As for what to do on the day... well, my 15th i wnt to a LAN gaming place and my 14th, i spent at home playing freelancer (Not a very fun experience, ever tried taking on the Rheinland fleet in a rhino? not fun). Just do what makes you happy, go for a run, read a book, call you gf (Be thankful for her, ive never felt love, i have no idea what it feels like. All i can say, be happy) and ahve a long chat with her.
Anything and everything, if you wanted to walk around the house nekkid for the day, then go ahead! might as well squeese some fun into you teenage years before you have to settle down and get a boring job!