I'm talking about gutters on the street, beside the kerb. I thought this was implied after I said "keeping your block clean by sweeping and mulching". You routinely see older people in Asia sweeping and raking a communal area if you get up early to walk. There's a (probably obsessive-compulsive) 60 yo guy a few houses down from me in Australia who might've retired early and now goes around raking verges and cleaning the footpath/gutters meticulously. Near my office, there's a woman who bakes bread for the joy of it and sells it at-cost via an honour-box in a sidestreet. She also turns verges and front yards (with owners' permissions) into a community vegetable garden. If others were given an opportunity equivalent to early retirement, these sorts of things might be more common.
As for why: for purpose, for praise, for community, for mental health, for trade/contribution, for skill building, etc. Loads of examples of this already. Maybe none of these things are attractive to you but I don't think that's universal.
Like I said, it's just trying to add to the default UBI, not getting everyone volunteering in their community or else.
Most retirees, early or not, do not contribute to society with their labor nearly as much as they did during their working years. What makes us think that UBI beneficiaries would be any different?
If the government gives out free money people will pocket it. Should not be controversial.