You could buy a piece of vinyl flooring or something similar to put under it? A shower curtain or table cloth would work too and than be folded up and put away when not in use.

My kids love when the water table comes out! I usually let them play just in their diapers or make sure they all have a change of clothes because they will get wet!

I also use my water table for sensory things like feathers, foam pieces, cloud dough, egg cartons etc. I will put small toys in there and let them play with a mix of things. I use macaroni and rice a lot though lol. I buy it with coupons or when it's on sale and can often buy boxes of it for 25-50 cents!! We finally did the coloured spaghetti and that was a huge hit!

Sand is another option. I'm a fan of letting kids be kids and getting messy but I will admit there are some things I won't do inside and sand is one of them. We play with sand in the summer at the playground haha.

I don't know if I have done it with my hdc but in the centres we did cornstarch magic a lot. It's really cool. Also, it's messy but kids love it....pudding and jello!

The pool with balls is a good idea! I did it with a playpen, maybe that would work for you too?