Most rental places are on Katella Ave. When me DF and I were there in May we got in an accident and ended up having to rent a car for the rest of our stay. We were at Jolly Roger on Katella and Harbor and walked to all the rental offices from our hotel.
Enterprise was the best! They gave us a great deal and even dropped us off aat the train station when we were leaving. We even got to switch cars half way through just to give us some exictment!