There is a fish hatchery along the Columbia River Gorge that is in a beautiful, green area:
http://www.oregon.com/columbia_gorge_attractions/bonneville_hatchery. I took my kids there when they were in their early teens, they loved it.
Herman the Sturgeon lives there, he's not the original but he was big when we visited!
If you plan on visiting in the spring, there is a beautiful tulip farm near Woodburn:
http://web.oregon.com/trips/woodburn_tulips.cfm. The kids also loved it, so did DH. Pretty spectacular display of tulips!