Another vote for tarp, bought at Walmart, etc.!
That said, I usually don't use it during surface intervals. I always come back to my car to change out tanks, and after changing tanks I just lay the rig down on the ground. All other stuff will be laid on top of my fins. (It's going back in the water anyway.)
Once I'm done for the day, I bring out my tarp and lay my rig down, and everything else goes directly into the car after a rinse. The real reason I need a tarp at all is to avoid getting dirt on my exposure suit as I take it off.