Is your small tree in shade or in the sun? Do you want annuals or perennials? What colors do you like?
Also, what is the tree? I'm sure folks on here can offer some great recommendations.
However, I find that I have asked similar questions here and of friends, then go to the nursery with a very specific list. But when I get there, I often just end up getting something that catches my eye, is on sale, and is recommended by the staff. Staffers at a good nursery (the garden addition to the big orange box store doesn't count) know best what is thriving and growing well and popular in your area.
Marigolds can be planted from seeds, are short, colorful and will reseed themselves for next year. Dianthas are also short and have pretty colors.