...is to drive to the water parks. Try this at home. Put on swim suit. Get totally soaking wet. Get into car. Crank up the AC to the MAX. Drive around for no less than 20min so the car can cool off to the hilt. There ya go. Got ch-ch-ch-Cher answer ch ch ch ch.
Even if you have a chance to dry off at the water park before the bus comes, there always a chance of rain you might not well predict.
If ya want to chance it, pack one of those silver plastic Emergency Blankets you see in the camping equipment departments. But nothing beats being able to turn on your own car's HEATER.