I agree with filling the spaces with annuals while your perennials mature. Looks like lots of sun near the trampoline. You may want to consider dinner-plate dahlias - big plants with big flowers and very easy to grow from tubers. Annual and/or perennial vines would look gorgeous along the wall of your house. Sometimes I put a big container of annuals in my flowerbed if I have a spot that needs a color boost. Adding some shrubs or roses will add focal points, texture and color.
When we moved to our house 15 years ago, there was one daylily plant. We had several years with a 8x10 trampoline and a 21' swing set. I'm sure your garden will "leap" in year three
