Apart from a tidy up you have great makings of a Japanese garden. I love your dwarf (maple?). Pick out some of those rocks and make a triangular arrangement (there is an art to this but in principle just play with a tall rock, a flat rock and a rounded rock to create an aesthetic grouping). Then plant around the base lots of ophiopogon (there are two types green and black). These are low growing grasses. In the back add more ornamental grasses that are taller: pennisitum is an annual, carex and stipas look lovely in a semi circle behind the tree and will give different height and colour throughout the seasons.

If you have a peat soil you could plant azaleas but frankly if you don't have peat soil then don't bother...
All the best for this lovely project

Remove the rocks that don't match each other: you have some sort of whitish granite-looking rocks and some darker ones that look like basalt. Pick one or the other. Arrange as ASVinteriors suggested. Finish removing green weeds and grass near the rear tree.

Take a look at English or Craftsman gardens on line. Walk around your neighborhood. Go to a neighborhood "garden walk" in the spring. You can choose perennial native ground covers from your local nursery; they will know what will survive the occasional snow in Seattle. Plant some bulbs in the fall.

In the meantime, try a few annual wildflower seed packets for this summer. Cover lightly with sacks of Miracle Gro planting mix (which is fabulous), water as directed on the seed packages if rain is not forthcoming.

Some solar "white light strings" could be really pretty on the tree, or more decorative ones if you like silly things, too. Small Japanese lanterns would be in keeping with the Craftsman style house; there are stone, cast concrete and metal lanterns available. Some can be wired with a tiny solar light, a bit of a fiddly job, but enjoyable.

Do you want to keep going with the Japanese garden feel? Inspiration pictures might provide some good ideas to start with moss, bamboo, dry pebbles etc...

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Thank you so much experts!

I would replace existing stone with three larger ones for a more dramatic effect. I always meet clients at stone quarry or nursery and let them carefully pick out a nice grouping of at least three boulders. Most stone yards will charge minimally to deliver and position stones with boom truck. The idea of using assortment of small to medium height grasses is great idea.