Just know that traffic in the Seattle-Tacoma area is crazy 24/7!! Use google for estimated travel time - it's reasonably close enough for estimating.
Port Townsend is a nice quaint small town on the waterfront with many old buildings and funky shops. You could get there on hwy 101 on the west side of Hood Canal by taking I-5 down to Olympia. Or go up hwy 16 up to the Hood Canal bridge. There is a ferry from Port Townsend over to Whidbey Island or go back over the Hood Canal bridge and take the ferry over to Edmonds which is closer to Seattle. Seattle ferries are way cool!
Go horseback riding on the beach at Ocean Shores.
Cape Disapointment State Park at the southeast corner of Wa. is interesting. If you have time, from near this Park pop over the Astoria-Megler Bridge to Seaside in Oregon. Neat little summer destination village with fun shops.
If weather is good, you could drive to Mt. Olympic National Park and go up to Hurricane Ridge (Mt. Olympus is 8K ft. elev.) out of Port Angeles. Great views from the Ridge.
A trip to one of the San Juan Islands (via I-5 to Anacortes) would be totally awesome but could stretch out the travel time for a day visit. Google says a trip from Tacoma to Lopez or Orcas Island is about 3 1/2 hours so is do-able. Highly recommended trip!
Consider visiting Mt. Ranier National Park to the southeast and driving the loop through the park. (Mt. Ranier is 14K ft.) Or a bit south of Mt. Ranier is Mt. St. Helens which is a really interesting visit as a result of the eruption in 1980. Also weather dependent for views.
Gotta visit famous Pike Place Market on the Seattle waterfront if you can. I'd also suggest Ivar's iconic seafood restaurant on the waterfront near the Market, but it looks like it is temporarily closed. Lots of old buildings with interesting shops in this old downtown area. Verry busy in the summer tho.
You might find a visit to La Conner (near Mt. Vernon) interesting - rather small "artists/craftspersons" village with funky shops. Somewhat out of the way tho., unless combined with something else.
Whatever you do visit, please report your experiences. ;)