Covered buckets or barrels would also work, especially if the sun hits them directly. They'll absorb the most heat, and release it quickest, if painted black.
But you should cover them or the warm water will tend to evaporate, waste the heat, and re-condense on everything.
The New Alchemy Institute on Cape Cod (I think it was them) suggested a double row of black 55-gallon drums along the North wall. They used big fish tanks with tilapia in some greenhouses.
Someone else suggested a row of discarded floorboard radiators (copper pipes with fins) along the ceiling to collect heat, and a pump to push the warmed water down into big PVC pipes under a concrete floor.
(Or run an electric heater for 5 minutes every 30 minutes during the coldest hours.)
