If the concrete is in good condition and it's nice and flat (not all cracked and fucked up) just paint it flat white. If not you can lay down plywood and paint that flat white, the only problem with plywood is you would have to find some way of securing it to the floor so it doesn't shift when you walk around on it. Never just put plastic down on your floor unless you intend on never wearing shoes in that room or ever EVER sliding anything around. You can rip the plastic really easy and its just a pain in the ass to maintain. Flat white painted concrete is AWESOME because when ever you need you can just hose the whole floor down (assuming you have drains, derp.) and it makes clean up a breeze.
Plus with putting down plywood you have the problem of mold. Any water that spills onto your floor will just sit sandwiched between the floor and plywood and you're going to be living in a gigantic science experiment gone wrong in no time.