For rainwater, I'd check the numbers on quantities of antifreeze that can be consumed by a human being or other anmal before there are any detectable (not detrimental, but simply detectable) effects on health, and then clean them sufficiently to insure that those amounts/concentrations can never possibly build up in anyone who eats the foods grown from them, perhaps even going so far as to apply a coating of a plastic sealer inside the barrel.
But I adhere to the basic rule of water safety, if the container has ever held anything not beneficial to human biology, DO NOT DRINK FROM IT. If it was only ever used to freight salsa or orange juice or chicken broth, fine, but not motor oil or anti-freeze or hydraulic fluid.
I'd say the best thing you guys could do with them is to punch holes around the periphery right above the base, weld on some artisticly bent rebar for legs to keep the base up off the ground, and sell them as garbage burning barrels.
Absent that, just drain them, pop the bottoms off, tip the barrels on side, squash the drum to half its original diameter, toss the bottom inside, finish squashing it flat as a sheet of paper and start stacking them up out back. Run a lottery for guesses as to how high the stack can get before it tips over. The winner gets the lottery proceeds plus the scrap value of the flattened barrels when they're taken to the scrap yard.