The world is a big place, so choosing what exotic locale to visit can be the hardest part of planning any getaway. At least as I see it, the point of traveling is to go somewhere else to see and experience things you couldn't at home, whether that's foreign cultures, unique cuisine, mountainous landscapes, ancient ruins, a tropical climate, or famous artwork. Think about what you most want to experience, and plan from there. Sometimes it can be helpful to pick a theme. If archaeology is your thing, you could plan a trip to Mexico that's focused on Pre-Columbian ruins (Chichen Itza, Palenque, Teotihuacan, and more!), then throw in a trip to the beach, a visit to a tequila factory, and a stop by the Museo Nacional de Antropologia--one of the world's best--in Mexico City. Are castles more your style? You could plan a trip to Germany, to see one of the fairy tale-est castles ever, Neuschwanstein Castle, and add in a few of the other plethora of castles there, plus visit the Berlin Wall, Cologne Cathedral, the Black Forest, the Berlin Island Museum, or drive along the Romantic Road--whatever piques your fancy. The purpose is to have a diverse experience, so, after you've chosen a theme, go ahead and sprinkle in some other activities that interest you so you can experience as many facets of the local culture as you can.
Also, keep in mind that inspiration can come from anywhere. Maybe you always wanted to see St. Basil's Cathedral in Moscow because it was on a round-the-world calendar that you had as a kid. Maybe you met a man from Argentina, and after hearing about his country, you really wanted to go. Maybe you've always wanted to see kangaroos and koala bears up close and personal. Movies can be a great source of inspiration--scenes from Star Wars and Indiana Jones were filmed in places like Tikal (Mayan ruin in Guatemala), Petra (fabulous carved stone city in Jordan), Arches National Park (USA), Tunisia, Sri Lanka, and more. If you love the landscapes in Lord of the Rings, you can check them out for yourself in New Zealand (and you can even see the set of the Hobbit!). We actually first started looking into Puerto Rico as our honeymoon destination because my husband wanted to see Arecibo Observatory, which had been featured in Close Contact and a 007 movie. It wouldn't have been worth it to go to Puerto Rico just to see the Observatory, but once I knew where it was I could research the region, and found all of the other great stuff Puerto Rico has to offer--forts from the age of piracy, bioluminescent bays, El Yunque rainforest, colorful architecture, and wonderful beaches, food, and people.
The first step in planning any trip is deciding where to go. Once you have this figured out, you can start to plan everything else--attractions, activities, lodging, transportation, restaurants, etc. Pick somewhere that's interesting to you, for whatever reasons, and then figure out what areas of that place to go and you'll be on track to having a fantastic trip!