I used bricks, the decorative ones that came in several shades.
I used 2 X 6 treated lumber on edge for my controlling edging. Then laid 3-4" of builders sand leveled and pounded as a sub-floor. Then laid the bricks inside in a herring bone pattern. Left over sand was repeatedly swept over the bricks filling in the tiny spaces and cracks really locking the bricks in place!
Underneath my benches, I used wood mulch. When I watered my plants, the wood chips or mulch provided wonderful drainage! The bricks were held snuggly in place by the wood edging and lasted perfectly for 30+ years.