Just south of the Rhododendron Garden [1] is Pacific Rim Bonsai Collection (253/924-5206, www.weyerhaeuser.com/bonsai [2], free), operated by the Weyerhaeuser Corporation; corporate headquarters is just up the way. The collection contains more than 90 bonsai trees, some more than 500 years old. Go 0.75 mile east from exit 142A, then a half mile north on Weyerhaeuser Way. (By the way, the correct pronunciation for bonsai—Japanese for a tree in a pot—is bone-sigh.) Hours are the same as the rhododendron garden. Lectures on growing bonsai trees are held April–September throughout the year.
Links:
[1] http://www.moon.com/destinations/washington/south-puget-sound/sea-tac-corridor/sights/rhododendron-species-foundation-garden
[2] http://www.weyerhaeuser.com/bonsai