I didn't do anything at all to celebrate my 18th or my 21st! I made sure my boyfriends 21st was unforgettable though. Me and his parents all got together and planned a beach party at the local pub. We decorated it, had special beach orientated food, everyone was told to dress up in beachwear and given a garland as they entered the pub. The whole thing only cost about �100. So worth it, we've got loads of memories now!!
If you don't want that/don't have time how about arranging a pub crawl in fancy dress with your pals? Take a trip to the nearest big town all dressed as 118 people or big pink rabbits or gangsters and moles and just get wrecked!!