I'm going to suggest avoiding anything to do with water - anyone suggests punting on the Serpentine for example, tell them do do one.. Is London the venue for anything London-specific or is it the location handiest for everyone? My stag do consisted of my local pub, a darts board, a bowling alley, the landlord being very generous, and then a house part across the road. It was a fine night and never entered the realms of silliness