i have a wire ledge that came with my cage and snaps in (i rarely use it because my rats have got the habit of just flinging themselves down from high places in their cage instead of climbing, and i don't want them to hurt themselves- i was also really sick of that clanging metal noise in the middle of the night). however, i do use it when it's hot- i put a bowl of ice cubes directly beneath it, so that the rats can lay on the wire and feel the coolness from the cubes without getting wet or getting the ice cubes all covered in litter. i also do the frozen veggies, and the fan. we don't have air conditioning.
oh, and i tried that bean-bag thing- i sewed a little fleece pillow and filled it with split peas, put it in the freezer- within two minutes my little olive had chewed a hole in it and pulled out many of the peas. the frozen bean bag appears to be a good idea in theory- anyone have any ideas of how it may work in practice?