if you want to go that route pack class 1 gravel in and tamp it flat and tight. provided you keep it dry it will be as hard as concrete and is what we put under retaining walls with an inch or two of sand over it. instead of sand i would use pea gravel. no sharp edges and enough agrate to keep it firm if it does get damp. just run tamper over pea gravel and pack it in. will give you a nice solid surface. sharp corners may still gouge if not packed tight enough.

class one has higher clay content and a little more sand with less crushed rock than class 5 gravel. the clay if kept dry will make it concrete hard.

added bonus is clay wicks moisture. if you do spill just add a bit of sand and hand pack it with a hand tamper. clay takes while to dry though but will return to concrete hard when it does.