I have a harness that I wet and put into the freezer. My dogs like ice cubes in their water. We also have a dog pool that the dogs like to jump in and out of.
Ice cubes with baby carrots frozen inside.
Shade hopping during our walks.
An oscillating fan at ground level.
When we ride in the car, Sidney plants himself right in front of the AC vent!
If the dog pool is not an option and it's very hot, I'll sometimes get a wet towel and use it to dampen his fur, especially his chest and stomach area.
Only way I know to wet a corgi: sit on ground, legs out straight, put corgi between your legs upside-down, pour copious amounts of water on the belly fur with massage. Otherwise, it's like trying to wet a duck. Keeps 'em cool. Without shade, overheating is a serious issue.
Play "Kill the hose stream". Gwynnie would rather bite the hose stream than EAT. It's a wet game. Never tires of it. It's like the vacuum cleaner. Lawn sprinkler? Children's wading pool?