Are you looking for plants that are flashy, functional, edible, both, or something else? Do you want perennials or annuals?
One that I would strongly suggest is chives. Edible leaves and flowers, suppresses grass to some degree, attracts pollinators, and it helps prevent
apple scab.
Another option is to let nature plant it for you, though I would suggest suppressing any grass invasion with a thick mulch. Most other herbaceous plants would be beneficial or at least benign.
If you end up planting perennials near a tree, I think that it is important to take into account the eventual size of the tree trunk, as well as the eventual shade that the the tree will cast. Planning for the mature trunk size will also help keep your tree from being shaded out by something like comfrey if you plant it far
enough away. I'd say 3-4 feet from the trunk is a good general distance for most plants. It's also probably better to add perennials sooner than later, to avoid damaging the tree
roots when you dig a hole for whatever you're transplanting.
Check out the book
Gaia's Garden by Toby Hemenway for some ideas on tree guilds.