A small hard-side is not a difficult tow. But there are minivans and there are minivans. An Astro/Safari with 4.3 V-6 can tow a lot more than an early 4-cylinder Caravan or a Mazda MPV, and there is a whole range of other vans in between. Astro has essentially the 1500 Silverado six cylinder drive train, which is fine for light towing.
I would put a transmission cooler on any minivan that did not come with a tow package (which sometimes includes the cooler but often a beefed up cooling system for both engine and transmission, plus electrical and suspension upgrades).
I'd be wary of towing much of anything with 2nd generation Odyssey, they liked to eat up their undersized transmissions even if not towing (my daughter went through three trannies in two vans).
A lot depends on which hard-side. The smallest A-Liners can be towed by a subcompact sedan.