For flooring I went to Home Depot and got 16 inch cement squares and put them down. It is going well so far. Otherwise I have dirt / mulch underneath.

I also have mine in the grass. Is the grass dead? Yes. Do you get to do some grounding while you sweat? Yes.

I recommend the bench from SweatTent. It's a great price and it's constructed well.

30 minutes of sweating is great! For seating, our bench is on sale right now (https://www.sweattent.com/products/cedar-bench)but anything wood works great. Flooring wise, there are some great options at Home Depot and Ikea. We recommend something that is able to withstand heat and moisture. You can also use it on the ground (concrete, grass, and wood decks being very common).