What is your favorite food? Can you make it or do you know where to get it? If yes, then your birthday absolutely should include your favorite food. Think of someone to enjoy it with. It doesn't have to be your favorite person, just your favorite person that you think will say yes on short notice. Contact them this morning, tell them its your birthday, and suggest getting together with them later so you can have some (that favorite food.) If you can easily afford it, it's a nice touch to offer to pay for theirs too, but if not, just tell them when you invite them that you'll each be paying for your own. That way they know before they answer.
The other 3 people who already answered also have good suggestions, but I'm a fan of indulging oneself at least a little on your own birthday. If you have time for both service work and good eating, then sure.