Waking up after a night of drinking will not be much fun. So, I suggest going camping and having a cook out on the Beach. Bring your Guitar and Sparkling Water.
skydiving ... don't do it alone though , it's more fun with somone you care about ;) !
well... let me warn you... if you go to a bar in seattle... and it's your 21st bday... they give you a pitcher with a big 21 on it... and everyone pours whatever they are drinking into it... and every year about 3 or 4 people die from alcohol poisoning... so take it easy.
oh yeah... and bring condoms!
Hang out with your friends and eat a cake.