Several items I've learned through trial and error. Sand is great, in a sandbox with sides high enough that they can't kick it out or mix it with other stuff. They'll need a short-step up to get in and out though. The sand can also be sifted then washed, then reused.
I have a vinyl flooring board propped up on a riser for them to rest their feet when they're tired of wire. Vinyl flooring is amazingly durable and waterproof and can be washed easily. Home Depot will sell single pieces so you don't have to get the whole box. A well placed plank will also serve as a step into the sandbox.
Those are my two thoughts.