Definitely stop at the border. They get testy if you don't.

After that, LaConnor would be a nice place for lunch and a stretch of the legs. I'm an airplane geek, so there's the Museum of Flight at Boeing Field in Seattle. Olympia for a look at the State Capitol. There are factory stores in Chehalis, but you may do better on prices in Portland, with no sales tax (if you plan to ship a bit).