I have lived in Portland since 1995.
I agree with a trip through the Columbia River Gorge - you may as well continue on to Mount Hood from there. The Coast is beautiful, but can be a bit foggy and dreary when it's hot and nice in Portland.
Mt St Helens is an hour+ drive and pretty cool.
I see some old school suggestions like El Gaucho and Jakes for restaurants. While these were the go-to places years ago, Portland's exploding food scene has left them in the dust. For steakhouse, go Laurelhurst Market. For tapas, Toro Bravo (the burger is actually the best in town imo) crushes it. Thai food is plentiful, but Pok Pok is the best (get the wings - trust me). Sandwiches - Lardo and Bunk. Quick breakfast - Pine State Biscuits (crowded on the weekend). South Park for seafood. Not a lot of good Italian here.
I would list brewpubs, but you really can't go wrong with any of them.
A nice way to see the Portland area is to do the 4Ts.
https://www.travelportland.com/article/4t-trail/
Downtown is nice, but the east side of the Willamette River is where it's at. For walking around, try NE Alberta St, SE Hawthorne, or SE Division - all between about 20h and 39th St.
Feel free to hit me up with any specific questions. September should be a nice time of year to visit.