Have friends over and just celebrate. Eat, drink, and socialize. I don't know if you live in the U.S. or not, but if you do most restaurants offer you something free on your birthday. Personally I like to go to wine or martini bars and just hang out with friends while enjoying some good food and drinks. Usually other people will hear it's your birthday and come up and say hi and buy you drinks. I'm a guy and it happens to me every year, if you're a girl like I'm assuming then you'll get more drinks than me.
If you don't drink you can still just go out and have a fun day. Go see a movie, go to a museum, take a small road trip. Go do something you've always wanted to do but haven't. Just make it a fun day for you.
Oh, and happy early birthday!