Haven't been in many years, but Pittock Mansion is a popular Portland attraction.
Downtown there are several old theaters and halls. The Crystal Ballroom is lovely, so you can see if there is any band/group playing/preforming there that they are intrested in hearing/seeing. OMSI's down there too, the museum building used to belong to the GE, so there's still color coated valves and an old generator in the Turbine hall, as well as the Oregon Clock... but beware, it is advertised as a children's museum (despite some of the the more mature traveling exhibits that stop there), so there'd probably be a lot of kids.
A nice thing in Portland is that there's the MAX electric train which can take them from airport, stopping close several hotels, at about $2 a trip, or $5 all day if they want to use the transit system to wander between the city's attractions.