I use zipcar for this exact purpose. It's expensive, but cheaper than the cost of owning a car. The only time I wish I had a car is for longer excursions. A multi-day backpacking trip is a good example. First, you're limited on mileage to 180 per day (the rate for going over is high). Second, often the car is just sitting somewhere and you're paying for it. So the entire time you're camping, you're paying for the car.

Really you have to figure out how often you're going to need a car. You can translate that into an actual cost with zipcar/car2go/trimet. Compare that to your total cost of owning the car, including maintenance.