Cooling Batteries

First of all, I'll let everyone know I am new to the forums here, and new to the hobby in general, so pardon my upcoming "dumb" questions...

I have a couple of electric cars already (2 Wheely Kings and an E-Maxx), so I've purchased quite a few batteries already. I read somewhere that it's recommended that to have three sets of batteries - one in the car, one cooling and one charging. That was great advice, except the batteries seem to take FOREVER to cool, especially on a warm day.

So, my dumb questions are:

How cool do they really need to get before charging them again?
Is it safe (ie, no damage to the batteries) to cool them off in an ice chest (keeping them dry of course)?
Any other suggestions on taking good care of them?

They're all NiMh batteries, I'm not ready for Lipos yet...