I live in Hillsboro, and the list of things I could offer is… significant.
So lets set some parameters: how much driving are you willing to do? What mix of indoor and outdoor activities? How much walking/ hiking do you want to do?
I just came back from a short camping trip to the coast. Right now is a great time because the resident grey whales are right by shore in Depoe Bay. The central coast is stunning, in general. Devils Punchbowl is super enjoyable. Those are all about 2-2.5 hours away, but well worth it.
You can also collect fossils at Beverly Beach right there.
Cannon Beach is a top destination on the coast, for a reason. Haystack Rock dominates the view, and provides some of the best tide pools. Its location as one of the quickest and easiest to reach beaches from Portland does help too.
Go to Mt. Hood. A ton of options depending on level of activity desired. The Timberline Trail to Paradise Park is great, there is Mirror Pond down by Government Camp, Zigzag Falls is an easy access waterfall in a canyon, Ramona Falls is one of the most spectacular hikes and falls if you’re up for 8 miles total.
As @ironwulf mentioned they just went to a congestion permitting system for old route 30 from Vista House past Multnomah Falls. You can still see Multnomah Falls using the I-84 center lot without passes though. However the drive down old 30 past Corbett and Latourell is fantastic and provides half a dozen great short hike options to various 200+ foot waterfalls.
Silver Falls state park by Salem is quite spectacular as well. A few great hikes, including a few that take you behind large waterfalls.
Powell’s is a no brainer. The Saturday Market by the riverfront is also a good visit. McMenimans and Deschutes breweries are personal favorites.
The Japanese Garden, International Rose Test Garden, and Washington Park in general are must see places. Forest Park if you want nature hikes right in the city.
Realistically 3 days barely scratches the surface, and if there is any specific thing you want in more detail, I can give it to you :)