Wood chips. Often available for free from arborists and utility companies, they are easy to walk on, look clean and tidy and can be reused elsewhere as a mulch when the garden gets its new landscaping. A couple of inches can cover a myriad of sins!!

Also, as you can see in the photo in the original post, there's a 12' trampoline that will be put back together. The contractors also extended the concrete patio area quite a bit. So the amount of visible ground will actually be less than what's in the photo.

I'd go with the wood chips also. Order bulk from a landscaper to deliver.

If you need something solid, you can check into outdoor floor rental. Many rental places have it- some is just grids, some get pretty fancy like dace floors.

RV patio mat? You can take it tent camping when you're done. Or, I guess, RVing. :)

If you don't rent a solid floor like bees recommends, you'll want to get rid of loose rubble, etc., that might trip partygoers whether you get a mat, mulch, or straw...

Thanks all. I think I'll go the woodchip route.

I can give you one more idea. You can temporarily cover the floor with artificial grass. As a result, you will get something like a garden party, like this https://adultpartythemes.com/ideas/spring-garden-party-ideas. Spread artificial grass on the site and leave it in this position for 10–20 hours. During this time, the villi will straighten, and the canvas folds will even out. Before fixing the edges of the artificial turf, ensure that there are no gaps between them and that they fit snugly together. Trim off excess if necessary. Then, you can attach the edges of the roll to the ground base using ordinary long nails, hammering them. Hats will not be noticeable among the blades of grass.