Sweet potatoes are my personal favorite ground cover, but they don't tolerate being walked upon. And they exude a sticky sap when crushed, so laying down on them won't be pleasant.
Perennial peanut might be a better choice. This is not the peanut that we eat and make peanut butter with. It's a fairly low ground cover that will take neglect, drought, and heat and keep on living. I've never tried laying down upon it while naked, so I can't vouch for its softness. Around here, laying naked on the ground is guaranteed to give you plenty of lava cuts, so we tend to use beach mats to protect ourselves.
I can think of a couple of other ground covers, but they have drawbacks. They tend to spread rapidly, are difficult to control if neglected, and aren't pleasing to lay down on. Another one grows 15" tall and will not stand being walked upon.
Clover and tropical grasses can be difficult to remove without resorting to either herbicides (nor
permie!) or something like long term use of thick black plastic.
Perhaps something like oats or wheat may do if you don't mind tilling the seed in and mowing it a couple times to keep it shorter.
Quite honestly, I can't think of anything that meets your criteria 100%. Perennial peanut is the closest.
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