Lots of very good recommendations.
To anyone wanting to stay near Seattle, the city is like an hour-glass, with the main downtown area at the choke point. Puget Sound is to the West and Lake Washington is on the East side, severely limiting camping opportunities close to the city.
Staying over near Pouslbo and ridding the ferry could be a memorable experience for a visitor not familiar with these larger "boats". Washington State Ferries are huge with restaurants and parking for over 100 cars. The ride can be an enjoyable experience.