I like to walk the dogs first thing in the morning before it gets too warm and try to wear them out then! I may do multiple laps of my normal route, or run instead of walk, because I know we won't be able to get out much for the rest of the day. Then I do another walk at dusk when hopefully it's cooled down again. If it's still warm at that point though I'll limit the walk. If it's really warm at my house, I'll even take them to the coast where it's typically cooler and we can get more of a walk in - but of course this is dependent on your location!
For the in-between time, I do frozen treats. If the dog is toy or treat motivated, you can freeze some water or watered-down chicken stock with toys or treats inside. Depending on your set up, this could be inside of a container (less mess) or just give them a big ice cube. I'll also freeze some of their food or treats inside of a Kong - adding wet food solidifies things and for big chewers, this can be a good time consuming activity. For my senior dog, I'll make pup-sicles in a cupcake tray. I puree baked sweet potato, unsweetened apple sauce and peanut butter, pour it into the cupcake tin, freeze then put it in his bowl. He'll lick at this for a long time!
In my experience so far this summer, most dogs just want to sleep when it's warm out. I'll encourage them outside a few times to at least stretch their legs a bit, but many want nothing to do with the heat, so I follow their cues!