Once you've chosen your destination and narrowed down where you'd like to focus your trip, the fun can really begin. I love planning trips, and researching all of the options and places that you can go is one of my favorite parts. A great place to start is UNESCO World Heritage Sites. These are sites that have been chosen by 21 UNESCO member states because of their great educational, cultural or natural importance to humanity, so you know they're going to be worth seeing! It's recommended to read a bit about each one to see if you'd be interested in visiting, and then see where it is in reference to the scope you've chosen for your trip. If it's out of the area where you'd like to stay, you'll have to decide whether it's worth the journey. For example, is Venice worth the five-and-a-half hour journey (one way) from Rome? You'll have to decide, and possibly adjust the scope of your trip accordingly.
Other great places to look are at national parks, top attractions on TripAdvisor, places listed on the wikitravel page for your destination, in guidebooks, or even from a quick Google search for "best ____ in ____" (examples: "best temples in Japan", "best nature reserves in Kenya", etc) to find advice from bloggers or other travelers.
Below are some tips for different "categories" of attractions. The more categories of attractions you have, the more diverse and well-rounded your trip will be. Sitting on the beach all day for two weeks sipping mixed drinks may be fun, but there isn't a whole lot of adventure in it. Travel and a vacation are not necessarily the same thing.
When people think of travel, many think of these first. After all, there are so many iconic ones--the Eiffel Tower, the Great Wall of China, the Pyramids of Giza, the Leaning Tower of Pisa, etc. Don't limit yourself to just the buildings that end up on postcards, though. Many places have beautiful palaces, museums, and temples or striking castles, cathedrals and forts. Some cities have scenic neighborhoods, like Old San Juan in Puerto Rico, Prinsengracht in Amsterdam, or the French Quarter in New Orleans. Wandering through these neighborhoods can be a delightful, free way to spend an afternoon and snap a lot of photos! Cities that have been around for a very long time often have multiple architectural styles represented, and some even have some surprising styles. Edinburgh, for example, is nicknamed "Athens of the North" because of the prominence of the neo-classical architectural styles and multitude of pillars on its buildings.
Many trips include some kind of experience with nature or the opportunity to view wildlife. National Parks (of any country) are a great place to start if you'd like to see animals or natural wonders. National parks are areas of conservation, so by visiting you're supporting that and you're likely to see some pretty amazing things--after all, people did decide that the area was worth preserving! There are some national parks in the world where it'd be hard not to see wildlife--Yellowstone or Everglades in the US, Serengeti in Tanzania, Galapagos in Ecuador, or the Great Barrier Reef Marine Park in Australia. There are others where you're guaranteed breathtaking sights, like Iguazu in Argentina, Sagarmatha (where Mt. Everest is) or Torres del Paine in Chile. But sometimes there won't be national parks, or if there are you may not be able to see very much wildlife there. Many animals are rare or difficult to spot in the wild, even if they do inhabit protected national parks. Many people would prefer not to visit zoos (my husband for example) because the animals are not free and in their natural, wild state. A nice compromise is to visit a wildlife refuge, sanctuary, or rehabilitation area. You'll still be supporting conservation, and though the animals are not "free" they usually can live in a pretty wild state. For example, when I was in Bolivia, we saw one wild monkey and a few different birds around the campus. But then we visited La Senda Verde Ecological Reserve, and we were able to see so many more--parrots, macaws, a toucan, monkeys of all kinds, tortoises, turtles, even a coati--and while they were not wild, many of them were rescued from poachers or from captivity, so they may not have ever been able to be "in the wild" again anyways. The animals still had free roam and were able to live in the sanctuary under safe and pretty wild conditions, and we saw far more animals in our few hours there than we did in the rest of our time in Bolivia. Your best chance of seeing orangutans in Malaysia might be at Semenggoh Orangutan Rehabilitation Center. You can see dozens of koalas at the Koala Conservation Centre on Phillip island in Australia, and you can see the elusive and threatened giant panda at the Chengdu Research Base of Giant Panda Breeding in China. As far as seeing wildlife goes, I would try a national park first, then a conservation or rehabilitation centre/reserve, and finally a zoo, if you can't experience whatever animal you'd like to see in a more wild state. It's generally easy to find places like beaches or forests for hiking in the guidebooks or websites, and it's definitely worth it to step out of city life when you visit a new place!
If you're visiting a capital city of a country, there will likely be all sorts of museums and memorials, and one of the best (and often free or cheap) ways of getting a feel for the history of a place can be a visit to its museums and memorials. For example, Washington, D.C. has memorials to many different wars, Abraham Lincoln, and more, all located on the National Mall, which also houses the Smithsonian Institution museums. These are some of the best in the world, all free to enter, and there's sure to be one you're interested in. My favorite, when I visited, was the National Museum of American History, which houses an eclectic array of American artifacts, from Abe Lincoln's hat to Dorothy's shoes from the Wizard of Oz to the original Star Spangled Banner to some of Elvis's guitars. Capital cities often have wonderful, free museums, but other cities often have fascinating ones as well. Think about the types of museums you like--historical, scientific, art, children's, etc.--and research if there are any in your destination city/cities. For example, I really enjoy art museums, so I always try to find one when I am researching a trip. When I planned our honeymoon in Puerto Rico, I found the Museo de Arte de Ponce and we ended up having a really great day trip to that part of the island.
Don't forget about the local artistic performances of a destination. When in New York City or London, it makes sense to go to a musical on Broadway or the West End. In other parts of Europe, a ballet, opera, or symphony might make sense. But keep in mind that beyond the Western world there is a whole range of different things to experience, from Japanese kabuki theatre or Mongolian throat singing to Argentinean tango dancers or a Capoeira performance in Brazil. Try to soak up some of the local culture of a place by going to a play, musical, dance, or other performance while on your trip.
Why not try to learn a new skill while having some fun on your vacation? Take a class in a local dance--like salsa in the Caribbean or river dancing in Ireland. Perhaps you enjoy a martial art or yoga, and you could take some classes in a different country, or even the country where it was founded (karate = Japan, tae kwon do = Korea, kung fu = China). Otherwise, if you love the cuisine of a certain place, like Italy or Thailand, you could take a cooking class there and have some new dishes to make when you get home! And of course, when traveling, it's always beneficial to either polish up your language skills with a class or tutor, or even just start to learn some of the basics of the native tongue where you're going.
You probably won't be able to avoid running into people selling souvenirs on the streets if you're going anywhere remotely popular, but wandering through markets and taking your time shopping can be a great way to get a feel for the place. Food markets are great, because that's where regular people of the place you're visiting often buy or sell their food. Pretty much anywhere with a reasonable climate will have food markets--even the US Midwest has farmers' markets in the summer months. Wandering through a market (like the bazaars of the Middle East/Asia or medinas of northern African cities) is sure to be exciting, and checking out the foods, artisan crafts, textiles, and other products of a place can really help you get to know its cultures. Make sure to plan some time for perusing
Factory or farm tours
Another great way to learn more about a place is to take a farm or factory tour. Ones which produce the local alcoholic drinks are often popular--like winery or brewery visits, or visits to distilleries of rum, whiskey/bourbon, ouzo, or sake--and may even include free samples! Visits to plantations that produce a popular crop, like tea, cocoa, or coffee, are also popular. Beyond the culinary world, you can visit the Ferrari or Lamborghini factories in Italy, Fragonard perfume manufacturer in France, or the Gibson guitar factory in Memphis. There are often also places where you can see local artisans at work, making ceramics, textiles, jewelry, or other beautiful crafts for purchase.