Sure, those people exist, that is fine. Some people do that now and far beyond their 20s: just hitch/bike/van around and do the odd job for a few days and live off that the rest of the time. There are communities where I live who allow anyone to come in and clean and cook together so you can stay and eat for $0. That is without basic income so it will happen with too. Thats a tiny minority. Most will get bored, loney, fed up etc if they even 'dare' to start such a life.

Ok, but thats "travel and adventure". Travel and adventure are demonstrably available to people who arent rich.

"Tiny Minority" until you no longer need a job to maintain the lifestyle. A non zero amount of people will go from not traveling, to traveling.

This is neither a criticism nor endorsement of UBI. Its just an observation.

Also, how is it bad to have such people? These are the people who go on to write interesting books or bring a different perspective to society than the one in which everyone lives the same life.

Indeed. No problem.