I just came back from Washington - it's a great place to travel. Here's what we did:
Day 1: Starbucks, Pike's Place Market, Beneath the Streets Tour (this was great, I liked the Underground Tour better when I took it last time), the Seattle Great Wheel, dinner at Island Crust and a Mariners game
Day 2: Chittenden Locks (absolutely fantastic, you can see lots of boats go through the locks and you are right up close - I'm surprised that this doesn't get more fanfare), Space Needle, Chihuly Gardens, Museum of POP culture (waste of money and boring - the building exterior is the best part).
Day 3: Shabbos - we stayed at the Four Points Downtown and it is NOT shabbos friendly at all unless you are on the first floor. Every staircase has light sensors. The only exit right now is the sliding front doors. The other exits are locked with a chain.
Day 4: We drove to Mount Saint Helens, walked around at the Johnston Ridge Observatory, went to Coldwater Lake, hiked part of the Hummocks trail (it's a fine trail, nothing too exciting) and then went to Ape Cave. Ape Cave is a far drive but is totally worth it. You need flashlights, your phone isn't really bright enough. Right now, you need a reservation ($2). We had a 5 pm reservation but no one was around to check our tickets. I am not sure if they were checking earlier.
Day 5: We drove to Mount Rainier and followed the west side loop drive until the Grove of the Patriarch trail (
https://visitrainier.com/loop-4-west-side-loop/) - the best stop was at the Henry Jackson Visitors Center. There was still a lot of snow and you can walk all over it. We also loved the Grove of the Patriarchs trail. After that, we drove back to Seattle to get dinner at Pabla. We flew out the next day.