I'll probably regret posting this. One of my favorite regular rides goes south along the shore of Lake Washington, usually to Seward or Beer Sheva Park. Sometimes I'll go all the way down to Renton and follow the I-90 Trail home, over Mercer Island. But a lot of the time I'll climb Beacon Hill, and this means crossing the Chief Sealth Trail, either right before Beacon if I climb Orcas, or near MLK if I'm further south.
The Chief Sealth Trail is almost always empty. In the two years I've enjoyed it, I may have seen a total of 20 other people there, and so I've renamed it to Empty MUP Lane.
Better yet, it's moderately hilly ( most of the hills are about 8 %, says my Garmin ) and there are a lot of nice S-curves. When the trail is empty - almost always - the turns are fun to navigate at speed. On a clear day, it's got an interesting mix of nice scenery ( the south end of the lake, and the Cascades & Rainier in the background ) and the local housing. There are some nice gardens in the summer.
This is a short, enjoyable trail, and I like mixing it into my longer rides.
Here are some photos of the trail.
Here's a map, although it's out of date - they've extended the trail from Beacon to 15th, where you have a view of the Olympic Range and part of the Sound.