Day trip suggestions near Olympia, WA?

In Olympia proper is the year-round, partially-indoor Olympia Farmers Market. It's pretty good, and in September will be overflowing with produce. In nearby Tacoma is the Museum of Glass, which is open until 5PM daily (including Mondays) and features glass-blowing demonstrations. Between Seattle and Tacoma is the Museum of Flight which is good for at least a full day. If you get to Portland, you could easily spend a full day at the Portland Art Museum - it's exceptional in scope and breadth.

Olympia (Washington) is a mid point of Seattle and Portland, so it's easy to make a day trip to either of these cities. Olympia is also just 2 hours drive from the Mount Rainier and Mount Saint Helens.  You could also go to the beach towns such as Westport (Washington), or Long Beach (Washington) in WA.