Pallets aren't an even or quality surface. They're made of the wood that's not good enough for anything else.
Make it like a deck... sink a few 4x4 posts for the corners, long sides, and center, level the tops of the posts build your box with 2x10s and 2x4s, with the "2" side up and enough of them to support the entire riding surface with no bounce, then surface it with smooth plywood. Then put a canopy over it, or cover it with a tarp to keep it from getting wet.
That's the "I have a budget" version. The "I don't have any money to do this" version would be different.