you could plant a hedge (of mixed or single species) of something(s) in the 3' height range. Shrubs or perennials. You could also plant some small trees (like crape myrtle) that would provide overhead sun protection without blocking the view. Things that flower a lot during the summer would be nice.

I would get some 4 x 8 lattice sheets, paint them like the deck, and cut it to fit around the bottom of the deck. You could hinge it for access, but it will keep critters like possums and racoons from crawling under there. You could go a step further by attaching hardware cloth to the inside to keep rats out.

You could then plant a variety of colorful shrubs and vines to beautify the area.

Most plants love water, so their roots will go under the deck to the wettest parts.

Suzi

For sure close it in. I would lay a garden hose around the deck to determine some curvy lines (please don't make straight lined beds), Don't fill it all in, do several groups with separation between them. I picture some ornamental grasses in various heights, with some splashes of colored perennials, depending on the sun exposure, which appears to be full sun, look for drought tolerant, which usually means they will thrive under neglect.However they all need some water and care. Most bushes, once established do well with very little effort. Maybe throw in some big boulders to take up space and they need no care at all. . . Who knows, after you put all the effort into it, you may surprise yourself and not have such a green thumb. :)