I always rent a car for one day to go grocery shopping. The cost is $25-$30 total. We have the ability to shop where ever we want. It also gives us the freedom to eat a couple meals off property thus saving us money.
So if you rent a car and it cost say $30. You can also go out for lunch and dinner off property. Think about it... It is much cheaper to eat off property so the money you save by eating off property for the one day you have a rental car, will pay for the car.
By the Winn Dixie there is a cii's cii's pizza buffet. The cost is less than $5.00 a person. Its all you can eat pizza, pasta and salad bar.
Where at Disney can you find a deal like that! There are many places to eat by the Winn Dixie which is a short car ride from Downtown Disney.
So if you take a cab your main focus is shopping at a store close to where you are staying to keep the cost of the cab at a minimum but the grocery store close by may not be cheap. I can tell you the Winn Dixie is expensive! So if you have a car you can go to the Super
Walmart or Public's which are not that far away.
Now to rent, both Dollar and
Avis have rental car counter at 2 different hotels within walking distance of downtown disney. Avis is at the Hilton hotel. I am not sure the name of the hotel where Dollar is. It is the hotel that is a cross from the downtown disney bus stop. If you are waiting for a bus it is the hotel right in front of you. The Hilton a 5-10 minute walk from Disney.
Renting a car is the best way to go.
REMEMBER you must tell them when you check in at the resort that you have a car even if you only have a car for one day. That way you can park at your resort with no problems.