We'll definitely be planning some sightseeing things. The ocean is two or three hours drive, so may be worth booking a place down there to stay overnight. It's not on my list of things to schedule visits to, but I can add it. How long do you think people will want to stay before and/or after?
I'm thinking Fort Vancouver
National Historic Site--the very first fort in the Oregon Territory (see, we were here before Vancouver, BC!) which has volunteers doing black smithing and tanning and cooking in the old ovens and all sorts of cool demonstrations. This would probably be about a twenty minute drive from the hotel.
And Multnomah Falls is a must, plus you get to drive along the Columbia River Gorge to get there, which is worth the trip all itself. It's one of the tallest waterfalls that runs year-round and is surrounded by a beautiful forest. You climb a staircase to get to a lovely arching bridge (my grandparents got engaged on it!) that lets you look up and down right over the falls. The adventurous can hike all the way to the top. It's about a 40 minute drive from the hotel.
Mt. St. Helens is amazing to drive up to the observatory to see the mountain up close, watch videos of the 1980 eruption, see the seismographs that are constantly recording the ground movement, and buy things made of the ash. It would be a little over an hour drive. (note, I took that picture about ten years ago when it was steaming. No danger of an eruption, but so cool to see.)