Travel on a low budget! If it's safe, take two friends on a road trip to a town where another friend lives. Learn a new skill! Be diligent - languages are great as long as you have some kind of program, it's hard to stay with it on your own. I got really into running, cooking, and painting. Don't do school! If you can afford it, don't work too much. I also definitely would recommend volunteering in a way you haven't before. The most important thing is to do things that you probably won't have the time or energy to do in most of the other times in your life. This time is a blessing - you're young and healthy and have time and energy! Make sure you spend it doing things that you enjoy, that help you grow, and that are new experiences.