Like we mentioned with the Seaplane adventures, getting up into higher elevations can mean cooling off as well. If you're close enough to do so, drive up into the mountains and find a nice spot to hike, picnic, or even just take pictures.
While Washington doesn't have the weather of warmer states that feature a significant number of outdoor restaurant seating areas, there are still plenty of dog-friendly farmer's markets that have shaded seating and fresh fruit juices and other cold drinks to help keep you cool (and nourished).
While Washington may not have the same number of lakes as some of those in the Midwest, it certainly boasts a significant portion of shoreline to enjoy. Find a lake or access to the ocean and go take a dip! Just make sure wherever you go is dog-friendly so you don't end up with a ticket or fine.
While Washington may not be hot year round, that doesn't mean that hot days don't happen frequently enough to necessitate a game plan. So if you find yourself Evergreen with envy over other states' abilities to handle warmer weather, don't forget that Washington can be the perfect place to find cool and unusual entertainment, whether you're just visiting or staying for the long haul.