if you like running I believe there is the Forest Hill Harriers - otherwise look at the newshopper website for activities. I think there are some martial arts classes around

unless you want to join the TA - they are based in Bromley Road next to the Police Station or I think there is a unit in Camberwell, check out the MOD website for units if you are interested. Or what about cadets/scouts/guides - I think they are short of adult leaders