Now, the fun part. Break out the maps.
Libraries are good sources of maps and atlases in a variety of sizes.
Make 20 copies of a small world map. Try not to confine yourself, or impose limits based on the areas you are familiar with. You may be surprised at how many places get added to your list as you become more familiar with travel literature. Take into consideration whether you will be planning adventures, like diving, rafting, skiing, desert treks, or rainforest hikes. The quickest and easiest way to research countries is to go to the library or bookstore and read the Lonely Planet's introductory sections.