The rims, obviously.
Then you may want to ask your Toyota dealership if there's anything that needs to be addressed in the vehicle's computer to ensure the speedometer/odometer and transmission shift points are correct. It will depend on if the outside diameter of the two tires are the same, or if they differ enough to worry about.
Just curious as to why you are wanting to go to 20 inch tires, are the 17 inch tires not expensive enough for you? If you went with 20 inch low-profile tires, I'd imagine the responsiveness of the steering would improve a bit as there'd be less sidewall to flex.