What should I do for my 18th birthday?

I'm turning 18 in November and I want to do something fun... I live in Northern California and all my friends live elsewhere but I still want to do something cool. We went to the Exploratorium in San Fran a few months ago so that's out. And I just don't know. I don't want to go and just party, I want to DO something. Do you think you can help me out?