It's really down to what interests you; what you'd like to develop, and what you'd like to try. Perhaps you're an energetic, active person who can't sit down for five minutes? You might want to consider an adventure or party trip spent travelling and having fun. Maybe you're a more laid-back person who just wants to chill and see some beautiful sights or laze on a beach? You might want to go on a round the world trip to all the world's major landmarks. You might want to get work experience and spice up your CV with some paid work. If you have a desire to help out in communities abroad, you could use your time to volunteer in another country. You might all be about learning new skills on your gap year, or you might have a specific goal in mind, like getting teaching experience. Alternatively, you could do a mixture of everything, and divide your time up how you want.
Some advice would be to not cram everything together into a very short space of time. You need to account for things like delays, unexpected route changes, and a whole host of other irregularities. Set aside more than enough time to do what you want to do. That way, you can be sure that everything will get done, and you'll be likely to then have some spare time that you can use to relax, or do something you hadn't thought about doing or visit somewhere you hadn't planned to. It's these kinds of things that make your gap year really interesting, and add a bit of adventure to it.
Ultimately, no one can decide what you do on your gap year for you. It's down to what interests you, but do consider doing something you've never done before - you might find a new passion, and it could make your gap year really memorable.