How about a sleepover at home for a choice few best friends? DVD, pizza and popcorn. Would avoid the rampaging masses taking over your house (just a rampaging few instead!).
Other good ones my son has been to are go-karting at the Latchmere, archery at the Aspire Centre on Merton Road, Laser-tag at Eddie Catz in Wimbledon, crazy golf in Wandsworth Park...You get the drift. Parties they rave about tend to involve sport and/or movies - boys' paradise, clearly!