There's a ton of other good food there, but I don't have any more specific must-eat places there since Seattle Deli closed during Covid.

One cheap activity recommendation is an afternoon ferry trip out to Bainbridge Island. There's a cute town on the other side that you can walk in to, but just taking the round trip ride is fun, and it's a beautiful view coming back into the city at night.

If you decide not to go to Rainier (if conditions aren't great), another trip that's about the same length is Deception Pass. Take the ferry from Mukilteo, drive up Whidbey Island to Deception Pass. There's places to hike and a beautiful vista heading over the pass (it's a waterway between Whidbey and the mainland). I like hiking around Rosario Beach area, but the other side is nice, too. Then you can either drive back down the island or drive through Mount Vernon and down I-5.

Enjoy! Seattle's a great place!