we used to watch the Seattle fireworks shows from the top of Mount Walker, just south of Quilcene. on a clear 4th it is an awesome way to view the light show...you could take in both the Lake Union and Elliot Bay displays.
of course while there is a way to hike up there, the easiest way to get there is to drive all the way to the summit, so probably not what you're thinking of :[]
many of the eastern oly mountains would probably be good though...brothers, constance, townsend, zion (or maybe zion doesn't have the prefect view of seattle)