Or other improductive things. The more hardcore UBI defenders talk about paying Malibu surfers to surf all day, or to write haikus. That's pretty much doing nothing at a productive level.
One of the problems of those studies are that they are temporal, and everyone involved know it. If I enrole you for two years in an UBI experiment, you'll probably not leave your job, even if you hate it. But if I guarantee you the same income for the rest of your life... well, lets take a look at how many people keep their jobs, and specially their boring/shitty jobs, when they win a huge lottery price. Usually they don't do other productive things, or even other improductive things: they tend to destroy their own lifes.
Another problem is that you have to take into account how slowly people internalize the new situations. E.g. when central banks started to use inflation as a source of government income, in the first decades it was "free". But people adapted and started to track inflation and price indexes to peg the salaries to them, "learning" how to live with the new situation.