If the heat wave comes back, grab the kids and head to Vasa Park.
This is a large private park at the south west end of Lake Sammamish.
It is usually a few degrees cooler than in town thanks to a gentle breeze (and the lake of course). There are many trees for shade, covered picnic tables, a kids volleyball/soccer "field" and a couple playgrounds.
The best part is the swimming area, swimming platform, diving boards and two small water slides. There is also a boat/jetski launch.
Keep an eye on your kids as the 15-year-old life guards can keep track of all of them.
They are sometimes closed for private gatherings so call before you go.