My first thought is to do a container planting here, if you are able to set something on top of this. There are books you can check out of the public library to give you creative ideas. The more I look through books, the more inspired I get to be creative! Hope this helps.

I think a container on top would look good too and the irragation box would look good as a base. It's also situated perfectly for a fairly large planter.

I've just got into container gardening and i love it. You can add aluminum cans to the bottem of container instead of rocks and it will be easy to move.
I've just recently found that you can plant alot of differant perennuals in large containers.
Right now i've got daylilies,sedum,peonies,yarrow,petunias,coleus,creeping phlox,and soon will have iris in large pots.

You might try container gardening forum for ideas if you decide to go that way.

ground cover

I'd paint it as close a match to your mulch as possible. If you can't paint it, make a shallow wooden box the same size and plant in that. Being shallow will ensure it's not too heavy to pull onto the landing. Plant hens & chicks, low growing groundcovers in it and maybe a few pieces of small garden art. Mona

Mine looked just like yours. Painted it with flat black paint (for plastics) and sure made a difference. Though mine is in the back yard by the steps and have now planted herbs there. Creeping thyme would work well.

A fake, hollow landscape rock. You could probably find some to resemble the high desert stones. Or another alternative would be to raise the mulch level to the height of the box, then lay flagstone in the bed as stepping stones, and one on top of the box...you could still move it whn you need to.