Hmmm many things! Besides the obvious answer of invest/save the money. Go to a used book store and buy loads of books. There are some places which offer old paperbacks for a dollar or two. Find some new authors or new genres or a book that teaches you something new. I recommend anything by Asimov if you want excellent science fiction (that's personally where I would spend my money).

If you play PC games then wait for the steam summer sale and buy 2 or 3 triple A titles for really cheap. You can easily get hundreds of hours out of them which works out to some great bang for your buck.

If you want something interesting and unique, visit kickstarter or etsy and find something that catches your fancy. I like investing on kickstarter, a good project will keep you engaged in its progress and is a good lesson in delayed gratification.

If you're feeling curious, buy some old electronics/junk/etc and take it apart. Give yourself two tasks, one, understand as much as you can about what you took apart and two, then build something new with the disassembled bits and bobs. Make some art or a flickering light or little gizmo that spins.

If you have plenty of 'things' already then give it to charity or the homeless. They will appreciate any amount and you'll feel better having given to those genuinely in need.