For some, that other thing will be another leisure activity.
For others, it will be something like what we now see as work in one form or another, whether that be helping people in ways that robots and AIs can't, doing research, or even just doing some work that robots can do, because some people like to do it, and some people like to have a human doing it for or with them.
For others still it will be art: painting, writing, creating video games, becoming an actor or comedian, whatever.
Humans are not well-adapted to having nothing productive to do. If a given human wants to continue to do purely leisure activities, it is often because they have been so exhausted and abused by our current system that their body and brain needs the time to heal.