UBI would likely only be enough for the barest of necessities of survival. Gardening in an empty lot would be an obvious way to supplement that income, with the same benefits of a gym membership.

The point of UBI is that via automation and reduction in cheap energy, opportunities for useful economic activity will be harder to find. In the best case, most people would have a mixture of reliable but marginal income (gardening) and high-potential but risky activity (inventing) with UBI allowing enough security to make crime a bad proposition.