Here are some I have actually used:
Homework Club: activities: doing homework, being friendly, eating, drinking (I have adult sims prepare food or set out a buffet table ahead of the meeting so that the kids can fulfill their needs while doing homework and building friendships)
The Cleansweep 7: 7 sims who meet at each other's homes to clean and repair everything, rotating homes
Theater Kids: Sims who meet to practice acting, speech and debate, charisma, comedy, and storytelling.
and I have to share this because it's well, funny:
Friends of Dorothy: "and by MLM we don't mean selling plasticware!" for single guys to Meet, Laugh, Mingle. Activities: Be Romantic, Be Friendly, Be Funny, Drink Bar Drinks, Dance. One note: a club in which all members are encouraged to be romantic with each other quickly runs afoul of jealousy and hurt feelings so you have to either take that part off and use it strictly as a social club to let them get to know each other better first, or you have to use cheats or traits to prevent jealousy because you can't keep them from flirting with each other indiscriminately which in the end, wreaks havoc. Havoc can also be fun though.