Mud is best experienced out in natural locations IMO, and I already have sufficient supplies in the house to do a bath of HEC gunge if the whim takes me. But playing with a large amount of something sticky is a thing I've yet to experience and would normally cost more than I could spare for a WAM session. So I'll go with treacle!
or filling up a bathtub with hundreds of little vibrators and getting buzzed all over every inch of the body! Add slime and it would be like floam except instead of little foam balls, tiny vibrators!
That would work too. It seems like there has to be some sort of national tragedy to invoke enough thoughts and prayers to fill a whole tub, so I'm changing my answer to money.
I've filled tubs w/ shaving cream twice, first time using 22 cans, next time about 15 cans. It was enough for complete submersion for 1 person. I'd love to fill a tub w/ enough for 2 guys to sink under together.