[Matt Navarre's recommendation]

Portland is one of the few cities that has rolled out a really great car rental service, called Car2Go.

Here are the hilights:
• All the cars are the tiny, cute, fun to drive Smart4Two.
• One-way rental, pay by the minute which gets charged to your credit card. 38 cents a minute.
• Never talk to an agent, don't make a reservation, don't wait in line.
• Rental includes gas, insurance, and parking
• Apply for membership card for $35, for lifetime membership. Works in all cities that have cars.

Here's the narrative:
So if you're walking around Portland, you see a white-and-blue Car2Go car, you walk up to it, wave your card at the sensor in the windshield, the driver door unlocks, you get in, enter your PIN code, start the car and drive for 5 minutes. Get where you're going, click button on display to say you're done with car, get out, wave card on sensor again, and doors lock. Rental complete, credit card charged for $1.90. As far as I know though, there are none at the Airport - you'll just see them all over the place in town.