Annuals under a maple tree?

Try using New Guniea impatience or non-stop begonia instead of the regular impatiens.There has not been anything mentioned if they are safe to use or not.

try Hostas, hardy growers, like it shady, bloom in summer

You may want to put in containers for planting. I've seen lovely concrete circular formed ones that would work.

selective pruning to open up the tree so dappled light can get through will allow you a bigger selection of plants to choose from. Try adding a layer of compost to the outer areas. Epimedium is shallow rooted and likes dry shade. The containers are a good idea.

I would go the hosta route and if you still want color do inpatients in pots among the hostas