Flight
Southwest and Alaska Airlines typically have the best rates.
House Or Rental
Portland does not have many hotels, except downtown which can be quite expensive. The best option is renting a house or apartment through Airbnb. Make sure to select "entire place" so that you will be renting out the entire house, having it all to yourself. We live in SE, in the Richmond neighborhood, and there are plenty of restaurants and things to do.
Rental Car
There are several rental car companies located at the airport (no shuttle necessary). Getting around town is pretty easy, but a rental car is recommended.