I use my birthday as an excuse to gather up all my friends, get drunk, and cause havoc somewhere more upmarket than we usually do.
Something kooky to do, too, is put restrictions on what people are to give you for a birthday so people get creative. Last year I announced that all presents to be given to me -had- to be made and not bought, and I got some awesome random stuff. This years £1 present rule was okay too.