First idea- I think the rocks idea would only work for walls heated by the sun. But if you use you outside walls it ruins the insullation you do have. If you windows are big enough and placed right, an interior wall in direct sun light might work. But this also slows the heating as much as the cooling. not sure if there would be a net-gain.
Second idea- if you intend to stay that long and you are not worried about payoff period, go all out the first time. Take down the drywall and the fiberglass. Seal the walls with an inch or more of sprayfoam. Build another 2x4 wall with offset studs inside. Finish with cellulose and new drywall. I have done just that in two of my four rooms on the main floor of my house and there is no perceivable loss of space and it's SO much warmer and quieter. I hope to do the next room this summer.
It is not as easy as it sounds to move the electric to the new wall if you leave up the drywall. You will likely need to extend the wires, that means unaccesable junctions in the old boxes, that is against code and bad form.
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