Planting trees might only be worth $5/hour for the local lumber company, but the existence value of a new forest to the population generally might be many times that.
I suspect that the amount of potential work that fits into this category is enormous.
Broadly speaking, though, I agree. This is one other case where we can do better - instead of constructing a "broken window" in the private sector and have the person repair it to keep them busy, directly utilize their labor for a socially productive task that may not be market-efficient.
But I'm not sure that there's enough of that kind of useful work (i.e. the kind that actually makes a difference) to keep everyone busy. It also feels like it's mostly easy to automate.
Perhaps we should not worry about that until all the useful work has in fact been done.