I like the allotments idea, if there are a few other families within bike range who would like to get their hands dirty. It sounds friendly and good for the neighbourhood.

Once he's figured out how much of the area should be wood and coppice, would your Dad enjoy exploring your region hunting for fallen seeds or seedlings he'd like to plant? Not all would grow on first (or second) attempt, but it would cost nothing and successes could give great satisfaction. In a few years he would have his coppiced woodland shaping up after all.

Daily gathering of fallen hazelnuts is a pleasant project at the end of summer, and they are very satisfying eating whether green or dry. Of course trees you plant in the next few years will need a while before you can eat off them, but they grow fast.