The Downtown branch of the Seattle Public Library has two electronic practice pianos.They can be booked one hour at a time.The University of Washington has practice pianos on the third floorof the Music Building on campus. The practice rooms have busytimes and they are actually for students use only. I would avoid using a practice room there during students busy study times.
There is also a piano available for public (appropriate) use at Evergreen hospital, in the lobby. There are guidelines for the use of the piano posted near the piano so make sure you can follow their rules before jumping in...