Be your own date. What is your dream date? A cozy evening spent on the couch watching old movies and eating your favorite take-out? A leisurely afternoon spent at the museum? Shopping all day? Dinner at the best place in town?
Your solo birthday should be about you doing exactly what you want to do, so whether you decide to stay in or go out, be sure to choose a fun or relaxing activity of some sort. Because the day is completely yours, you don't have to worry about accommodating anyone else's tastes or preferences!