Grass.

I'm trying dutch white clover, low growing, nitrogen-fixing, beneficial insect attracting...how can i go wrong?

My plan is to use woodchips to begin with and let them break down for a few years, and then replace with grass at some point?? Let the tree get some size on it.

I'm guessing with grass you probably have to water a little more often?

Big mistake to plant grass or any other water hungry plant under your fruit trees. Every tutorial I've ever read on fruit trees says to keep the ground under the drip line clear (an inch of fresh compost every year is the way to go). If you've seen other recommendations I'd love to take a look at the link(s).

It also depends on whether you are aiming for optimum yields or just want some home-grown fruit. My greengage plum, for example, is perfectly happy growing up out of a bed of mixed perennials. I expect it could produce more fruit if pampered, but it gives me as much as I need. Fruit trees can be treated as useful but ornamental additions to the normal garden if you are pushed for space. Mind you, there is plenty of water available round here.

I had a neighbor who had pear trees that were part of her walkway (like what the city does with sidewalk trees). I'm guessing 3ft circumference with the sidewalk being concrete. It must have worked because she was using sticks to hold up the branches due to all the weight of the fruit.