Activities in general are a bit weird and in need of some TLC. They feel more like a hassle than a boon most of the time. As some (including myself) have reported several bugs regarding them, they are also not very helpful compared to other things you can do in the game. If you solely want to gain stamina through them, you have to wait for the soft cooldown of the spa treatment to wear off, or it's not as effective. The performance money gain doesn't scale with the amount of idols you have, while still increasing in stamina cost for every idol you have, and promotion is borderline mandatory early game to gain any sort of foothold.
Again, it's just my opinion, but I think the activities are in need of a bit of a rework. Sadambober, perhaps you can have a look at it at some point?