Rent an 18 video to watch with your friends. Organise a photo taking session for you getting your adult library card. Buy a set of kitchen knives. Wrecklessly throw fireworks around.
im the same as you my bday is next monday and outta my group of friends im one of the oldest, its really anoying cos some of them cant get id to go clubbing so ive decided on goin out for a meal and then onto the pub after where its easier to get served. hope u find something to do soon!! x
Bowling, cinemas, just drinking in someones house, plan something for a bit later - like a joint xmas and birthday party, paintballing, laser quest, swimming sort of party thing - in one of these massive swimming centres like in Blackpool and Rhyl, a day trip out somewhere random on the train/car/bus, theme park, beach day out (even though it's cold lol), a nature reserve place, ice skating, museum, picnic in the park, tie it in with halloween and have a house party (fancy dress of course), go the pub before under 18s have to be chucked out and laugh when your mates are sat there with coke and you can have something stronger, go to your local big city shopping, take a tour bus around your local big city, visit local landmarks, have a few friends around for a movie night, have a massive sleepover, camp in a random field - be careful you don't get caught though, go round town and see how many places your mates can get in, go out for a nice meal - some local pubs have brilliant food, or somewhere like Wetherspoons or Yates, go the sealife centre, go Madam Tusaudds (or however you spell it), go to see a West End production, go in a hot air balloon/car racing/etc (there are some ideas in the argos catalouge)
Hope there are some decent ideas amongst this lot for you Have a great day