Rocks need to be grouped. I've posted some pics from my garden. I live in TN, but maybe it will give you some ideas.

Regards,

K4

Yeah, a rock or one of them sculpture balls. A rock would be cheaper. What the ball has going for it is you can choose the color. You've got red leaves and green/yellow leaves on these 2 cultivars if my memory is correct...so, a blue ball or whatever color you like.
If you want some plant show between the trees, try some bulbs that like sun in winter and shade in summer like your scilla peruvianus or similar bulbs. Very easy and showy in Spring while your maples leaf out. Not much in Summer though.
Hellebores grow good in the Bay Area; small carex ornamental grasses might look good; just a few ideas.