It depends on where you stay and what you want to do!
Our recommended neighborhoods are very walkable. Public transportation is readily available in Portland, as are Uber and Lyft. There are also various scooter and bike rentals (such as Lime, Nike Bike, etc.)
However, if you want to travel outside Portland to visit the Gorge or the Coast, you'll need a car. If you want to rent a car for just a few days, you may find rates are cheaper in the city itself than renting one from the airport. We've had good luck with Enterprise, which has many locations throughout the city.
We lived in Portland for 4 years without a car. But if you are short on time and want to see as much as you can, it's certainly more convenient to have a car.
Parking: Most neighborhoods East of the river have relatively easy, free street parking. If you stay West of the river, plan to pay for parking.
