So, I knew about Earthworks Park in Kent, which incorporates the remnants of the original Mill Creek that once ran off East Hill and across the valley floor And a few years back I discovered the standing stones of Tukwila (or possibly Renton), near the remnants of the old Black River (now a riparian forest). But until Janice took me there I today I'd never so much as heard of Robert Morris Earthworks park.
An old gravel pit converted into grassy tiers evocative of Machu Picchu, or perhaps an inverted ziggurat, it's immanently walkable, so long as you don't mind choices restricted to (a) down and (b) up. You can even see Mt Rainier from the rim.
I'd definitely go there again.