That's definitely one of the biggest arguments against UBI, but I'm in two minds about it. I think you're right when you say it's in our nature to be useful in some way, and there's no doubt many, particularly the older generations, would struggle to adapt to such a change in their lifestyles exactly like you say. I don't know if it comes down to whether someone is a creative type or not though, personally I don't consider myself a creative type, but I'm pretty good at keeping myself occupied. I've also met very creative/artistic types, who without some form of forced structure, just can't do it under their own steam.

The way I see something like UBI being introduced, at least here in the UK, would be through something more like the negative income tax mentioned above. Another argument against UBI that I've come across is that if everyone gets UBI, then it doesn't address wealth inequality at all, so that problem could be address through a negative income tax system. It really all depends on your point-of-view really, it's an interesting topic. Arguments can be made for and against UBI from both a Right-leaning or Left-leaning ideology.