Growing up, every place I'd go for a kids birthday party, most namely Chuck E. Cheese, would have one of those ball-pits. The problem with them is that one kid at every party who decides to pee himself in the pit. They aren't easily cleaned, so they stink, not to mention the germs.
However, thanks to some genius we may never need to install another ball-pit. There are now giant indoor sandboxes instead!
This includes a new one in Denver called Kids Dig. It's an 85 square foot sandbox with more than 40 tons of sand, and all the toys you'd ever need.
There is a rule that your kid has to be potty-trained to go in, but even if they have an accident, you just scoop the contaminated sand out, and BAM, no pee stink or germs!