Good for you, you should treat yourself special on your birthday.
Others have had good ideas. Since your DH is working, I like the idea of getting together for lunch with girlfriends if possible or DH if he gets lunch hour off. The idea of doing something just for you is fun too. Also take the day off from cooking dinner, cleaning, shuttling kids to activities, and anything else part of the daily grind. Probably a few things I would like to do: go to a salon for a trim (or a total change), have them blow dry and style it, get a manicure, take a class, buy yourself a new outfit, lunch at a new restaurant (or a favorite restaurant), check out a museum or store you have passed but never got the chance to visit, go ice skating, or just stay in, watch movies, and eat junk food.