I agree that it really doesn't need a border, unless it needs to be bigger or you really want to put one on. I would go with a solid, at least for a first border, then if you want to do a piano key or stripe on the diagonal, it wouldn't run into the blocks. Finish with a third border in a solid to "frame" the piano keys, then maybe do the binding as a stripe on the diagonal or a pieced binding using various lengths of the fabric you used for the blocks. I am a big fan of pieced bindings.
Whatever you do, it's a lovely quilt. Oh, my choice, if it's for a girl, would be a solid lavender/purple.