Re: cooling stuff off in a toilet tank:
Most toilets that have tanks use a ballcock that is designed to allow a very small trickle of water to constantly go into the bowl to avoid having the water stagnate.
As long as the water is coming from a cool source (i.e. an underground well) this will keep the water in the tank somewhat cool, even if the toilet is not being flushed (and much cooler if it is flushed frequently).
I would never put food or drink into a toilet tank because:
1) If the top of the bowl is above the bottom of the tank, you can get water from the bowl going back into the tank (this setup is uncommon, except for fancy one-piece units).