Peas/sweetcorn or other treats in ice cubes, frozen peas/sweetcorn as they are (rinse a bit first so they don't stick to their mouths), frozen bottles of water in a sock (again so they don't stick to it) for them to lay on, I've seen people suggest slabs of marble for them to lie on... erm... that's about all I can think of right now.
If you don't have air conditioning, you could try keeping the windows and curtains/blinds closed in that room during the day and just open them at night to get some air through.
I use small, clip-on fans that I direct onto a hammock in their tanks. That way, they have the option of sitting by it or not. I was worried it would blow the bedding around, but it seems to be fine. They love it and always sit by them when they're on.
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?
That bean bag pillow sounds like a great idea.
I'd say the best bet is to try and make it out of a fairly durable material and sew two layers.
Like make the inside out of denim (or some other strong fabric) and put a flannel piece on the outside, or vice versa. Depending on what you really want it to feel like for your ratties. You could even use two pieces of denim. It's a fairly durable material, you can even use old jeans to make it!
I freeze veggies in ice and give that to them, or make cooked for them and chill it before serving. Draping a damp towel over their cage and putting a fan on it created a 'swamp fan' and will lower the temp my several degrees. Also, a terracotta pot or plate is awesome, because once wet (soak it in the sink for an hour or so) it stays cooler then the ambient temperature.
That chin-chiller thing is cool, literally and figuratively - and I've found slabs of marble at the thrift store for $1 that have plastic around the edges, originally marketed as cutting boards. I bought one recently and pulled off the plastic, making a chin-chiller on the cheap. I've seen them pop up at thrift a lot here.