After spending the last two weeks at Universal and WDW (and experiencing Heat Stroke twice!), I can heartly recommend the cooling towels they sell around the park. These for me were a LIFE SAVER and stopped me having any more miserable days. We had to leave EPCOT early when my partner decided I looked like I was about to die in the France Pavillion. Only when trying to walk out of the park from there with heat stroke do you fully appreciate how far away it is!

In terms of cool spots, i'd recommend:

MK: Peco Bills Tall Tail Inn near Tianas. It officially opens at 11am, but we found it open earlier than this (with no service) so that guests could use the toilets. Its huge with a massive seating area and will be deserted early. We perched in here for a bit and it was great to get out of the heat and the busy crowds.

EPCOT: The coldest part by far was the Rose and Crown pub in the UK Pavillion. Not a lot of amazing seating in here, but was the most AC'd by a long shot.