One of the things I've been playing with is water as thermal mass... though the filled Arizona bottle collection only approximates a 10 gallon fish tank, and some of them have been dyed black, since it was more of a concern to try for a heat collector in winter.
It sort of helps, but the problem is keeping the water shaded, since once solar-heated (of course, turns out the shelf I'm using gets more sun in summer than winter) it takes a cold day to bleed the collected heat off. [If you're a true hippie, you want something more like 55 gallon drums or repurposed chunks of conduit, with anode rods so they don't rust out, and a floor strong enough to support them.]
Another interesting thing to note is that external shutters/shades/blinds are infinitely more efficient than the internal versions, which tend to act as solar heaters in the gap between themselves and the glass... I've toyed with the idea of cutting down the perforated shields they put over fluorescent lights in drop ceilings (the sort that are basically a white plastic grid, deep enough that, hung vertical, the edges would shade/reflect away a good chunk of incident light) and suction-cupping them or hanging them in front of windows outside, which might not look so bad, while cutting down the amount of energy that makes it in by a good fraction... Never managed to bother implementing it.
