In general, I rely on air-conditioning since I like the same temps as my newts, but I did have this concern when traveling with 3 red efts this summer; we were away from both home and air conditioning for nearly 6 weeks with no fridge room.
- I downsized their quarters from a 10 gal. tank to a large "critter keeper" (perhaps about 5 gals?) for convenience of travel,
- used mosses/plants which would tolerate very low light conditions,
- made sure there were plenty of hides, including some that went down into the substrate which would remain a bit cooler than the air, and
- made sure they were very well ventilated and in the shadiest/coolest area I could find.
- On the days when the terrarium thermometer went above 70 degrees F., I used the frozen bottle method advocated by Jennewt. This would bring the temps down several degrees, which was sufficient for my situation.
I found that method effective for short-term heat, though it did require fairly frequent maintenance of the bottles and wetting from the condensation was rather copious (think wet, rather than moist). Never had a leaking problem.
Car travel was the trickiest for me. The critter keeper fit very nicely inside a cooler, and with a number of small icepacks/frozen bottles, I was able to keep the temp in the cooler--even in the blazing sun--down in the 50s for all of our 5-6 hour roadtime, with no change of icepacks. Because of the sun, I covered the cooler with a sleeping bag for added insulation. The efts were happier than the humans since the car ac wasn't working!
I should think the latter method would work for extremely hot days and heat waves in the absence of air-conditioning and if you have no fridge room (or if the family balks at critters in the fridge). It also has the advantage of holding the cold longer than just a bottle in the tank, so would be good for someone who cannot frequently change frozen bottles around during the day (working, etc.). Obviously you need to ventilate the critters once in a while and change ice. Lack of light is also not great for the plantings. Condensation builds here, too, so watching for wet conditions is essential.
All that said, dorm fridges are on sale in many places now that the college kids are moving in--a dedicated fridge would be amazing--and from what I've seen, these are much cheaper than the wine coolers a number of folk advocate.
Advance planning is a good thing.
DDiS