This has been an excellent read, thank you for all the ideas!
I always have a club per household, just to help with skill boosts when they get the perks they want. Then I have a club per career, sometimes two (like entertainers and musicians are separate, same with chefs and mixologists). I've set them meeting in fitting lot types, like athletes meet at the gym 'n do gym things, writers meet at the library and read and write, secret agents meet at night clubs and have drinks and talk to people, and so forth.
Then there's also special clubs, like Space Changelings that is for the sims whose parent was abducted by the aliens, resulted into an alien child being raised by a human (or a vampire). They use alien powers in their meetings, figuring out their heritage together. I also have Gamer Geeks (both videogaming skill and being a geek) meeting at an internet cafe, a voidcritter club meeting at the same cafe in the voidcritter arena room, and such.
I should really set up a club for elders, 'n more kid/teenager stuff.
I really wish they'd add dog/cat related things in the activities or requirements, and also more skill-level-based requirements for the really elite groups. Skill level 2 can be almost an accident.