use curved landscape border, fill in with dwarf shrubs, ground cover, steeping stones and mulch/rock. Only rocks for the stepping stones, bricks instead of steeping stones and a potted plant where the maple tree is.
Keep the garden vertical. That's why it's good to look for shrubs and trees that max out interest as they grow up, not out. Try dwarf varieties for a small backyard, as well as more columnar evergreens (bonus—they boost wintertime interest, too).