If you have a group of friends (even a small one) that you could convince to dress up, I would say you should throw a theme party! Picking something simple where people probably already own something to wear and would only have to pick up a few accessories would be simplest. Failing that, you could have a few friends over to watch old Disney movies/action movies/horror movies and just have a cheesy slumber party sort of thing.
If you're celebrating by yourself, you could try picking up a favorite craft or art project that you used to do as a kid and relive the good old days. Or you could pick a new skill and resolve to develop it throughout the year, which would give you something to work towards for your next birthday. Maybe cooking yourself something special would be a good way to celebrate as well - if you learn to make something that you really love or have always wanted to learn, perhaps you'll make it again some time in the future! Taking time to yourself to try new things is a great birthday experience, in my opinion.