I'm flying domestically and have a four-hour layover at the Portland (Oregon) airport.
Is that enough time to get into the city and back in time for my flight? We'll have backpacks, no suitcases.
The Red MAX takes 44 minutes from Pioneer Square to the airport. Source: Trimet.
So that's 1.5hrs.
Security wait times you can predict from http://www.ifly.com/portland-international-airport/wait-times . Let's put that to 20 minutes.
Actually you need to get off the plane and get out. I'd put that to another 20-30 minutes.
We are at the 2.5hrs mark. You need to wait for the Max, walk in the airport, your layover is a bit shorter because you need to be there by boarding and not leaving. So, you can make it to Moonstruck on Alder street, have a decent hot chocolate and then go back. Sightseeing, not so much. I certainly would rather than sitting at the airport. Moonstruck hot chocolate is one of the better ones on this coast, you need to go as far as Vancouver, BC to have better. (Or if you have broader horizons, Eataly in New York is best.)