I think UBI would be unsustainable without additional investments in education. If you are going to be taking public monies, that time should be used to educate oneself or provide some form of public service.
"We" don't do anything with people - we don't own them. They do whatever they want to do with themselves. Maybe art and crafts. Maybe some meaningless but interesting research. Maybe just live a life of pleasure. I suspect there'd be a lot of different answers. None of them are wrong.
Why do you have to do anything with the people? Certainly they will find something to do themselves, either hobbies like gardening or more serious endeavors like trying to establish a self-sustaining colony on Mars.
As others have mentioned, I think requiring education would be a bit of fighting reality (with eventual automation outperforming humans in many tasks). I think, as in the current world, some people will naturally choose to produce and contribute while others will not. Largely this is okay... (open source software, wikipedia, school PTA groups, hacker news, ...). Encouraging people to contribute to society seems like an undebatable good thing, requiring them to do so, not so much.
