If you are looking for a nice easy walk on a peaceful lake close to Seattle, I highly recommend Rattlesnake Lake near North Bend.
There are several short trails along the east side of the lake that are great for walking your dog or a quick picnic stop. On my walk there a few days ago, I only saw one other party walking around the east side of the lake. When I first saw the lake years ago, I was quite taken aback by the deep turquoise-colored water and the jutting tree stumps especially along the northern shore of the lake. Amazing to see such natural beauty just a half hour from Seattle. The old growth tree stumps and old foundations from Cedar Falls can be seen when the lake is low.
You can also catch the start (or end depending on your direction) of the Iron Horse Trail (the John Wayne Pioneer Trail) at the lake.
The trail to Rattlesnake Ledge, the prominent rock outcropping overseeing the Lake, is two miles to the top with an elevation gain of 1,160 feet. If you hike up Rattlesnake Ledge, be careful as a hiker had a fatal fall a couple of weeks ago. Make sure your kids are close by on this hike.