Agreed, you need a few trees. The plan and pictures together help make the existing space and elements comprehensible. But the plan geometry needs some work. There doesn't seem to be "theme" running through it. Below are some rough suggestions of how you might lay out. The corner is too important for planting to put the trampoline there so I've moved it. I would think all the trees would be fairly small, some narrow, some cone-shaped evergreens ... whatever works the best for a particular spot. Small trees are often made from large shrubs. "Would like ... enough grass to enjoy too." I left enough for rolling around naked. Is that adequate? The goal of having mulch alone where plants will grow eludes me. Why would one want the burden of top dressing annually and the look of parched desert? Just grow a groundcover in those mulched areas. The look is infinitely better and no need for adding more mulch ever. If you control weeds from the get-go, once the groundcover grows in, it is not difficult to maintain. The only reason to have some sort of gravel is for walking. At the firepit area you could use gravel or a hard paving, like brick, pavers, etc.

My suggestions would include: daylilies, 'hot lips' sage, salvias, shasta daisies, catmint and lambs ear. For evergreens try rosemary and sky pencil holly, and lavender if it grows in your area. I don't think these would litter too much. Include some asters and blue mist shrub for fall color, and some spring bulbs like crocus, iris and daffodils for early spring color. If you have some shady areas, you may want to try gardenias, hostas, pulmonaria. I'm sure others will have lots of suggestions. I just tried to think of things that didn't have many falling petals or leaves.

I've just put in a 54" above ground pool. Now I'd like to plant some shrubs - especially to hid the filtration system but not too large. The site is in partial shade. Any suggestions?

Hopbush is widely used as a hedge because it fits the job description for a desert hedge plant so well. Saying it's "over planted" here is like saying boxwood is over planted in England. Native to this area, deep green, evergreen, thornless, not toxic, vertical growth habit, and easy to keep in bounds ... as a landscaper, I'm in love with that list of qualities. It will grow quickly if you water it deeply and regularly, and then you can slow down the growth by reducing the frequency of the watering.