UBI doesn't mean "no work". It means divorcing work from remuneration. There's still plenty of good work, but no one wants to pay you for it, so you get paid independently of your work. For example, you write songs. You build a house for yourself. You learn to sail, and take people sailing. You raise a kid.
You don't expect to get paid for any of it, but you do the work because work is meaningful and life affirming. Remuneration, the money you need to live, doesn't depend on renting your labor out, which when you think about it is a pretty modern idea. Back in the day we had patronage, and inheritance, and allowances (and slavery!) and all sorts of mechanisms that didn't directly tie labor to money.