It would be good to spread the sand out if possible maybe on a tarp of some kind. Or maybe you could put some in the lid and leave the rest in the sandbox. I'd cover it at night to avoid nighttime moisture collecting on it. (Also keep cats out.) Of course I don't know your weather and how sunny or dry it is during the day. Also, I'd keep turning it with a shovel some tool. Otherwise, toss it in the garden and replace it when it's time.

Because of that I never had a sandbox with a "floor"...just sides and the rain just drained away.
I would think that sand will be moldy and musty, unless is is very new and would sprinkle it on the gardens and get new for the kids to play in.....and punch holes in the bottom of that sand box!
Linda c

I don't think that it's really possible to clean the sand itself, once it's 'swampy'. I wouldn't trust it as a place for the kids to play in anyway. Too much potential fungal growth.

I'd dump the sand in some obscure garden area, clean the thing out with lots of soapy water, dry it and put it away for spring if this were me.

Wow....talk about dredging up old posts...in 12 years those kids using that sand box have likely graduated from college...maybe even with a degree in geology.

haha yes 14 years now but im here in 2020 with the same issue!! so im glad i found this lol

An oldie but a goodie. Same problem today — July 4, 2020!

How much does sand cost? Dump it on the garden, unless you have sandy soil, the garden will thank you, and buy new.

If that was a serious question, a 50# bag of play sand is about $5 at Home Depot. Probably cheaper yet at Walmart garden end.... ;0)

read the warning labels of cancer on those home depot ”play sand“ you need to buy higher quslity toxic free sand.