I have a youth club in Oasis Springs which I have had for ages and it works as a generic lot.
Generic is a good idea BUT it won't spawn any random sims to visit the lot. It will only be your sim and who they travel with.
With cafe, you will find most people will mill around the coffee bar/service counter and not use anything else. Similar for the gym, people will generally use just the gym equipment.
Spa is nice but you will find sims get obsessed with the lockers and changing into towels constantly.
A pool might be a good way to go. Sims generally gravitate to it but will wander round using other stuff too. Obviously most people will be in swimwear though and weather, if you have seasons, can have an impact.
Basically, I feel your pain here. Having a multipurpose centre is hard to classify but I love the idea and what you've done.
You can certainly change the lots function/type as you go, however, doing this while in play mode while the sim is visiting can be a little tempremental.