When to Go
Late spring ~ early fall (to avoid the winter seasons and the snow, go sometime in May through October, but also avoid the high heat in peak summer)
Things to Do
Enjoy views, strolling, see wild flowers, explore nature, etc.
Tips
1) It is a 3 hour + drive from Seattle, so if your family is not used to spending long hours in the car, make it an overnight trip. If everyone is okay with the drive, depart early in the morning, spend a couple hours there and come back all in a day.
2) Smith Creek Viewpoint is a nice stop for the whole family to take a short stroll and stretch out your legs from the long drive.