Daily cart rentals in the parks are $40 with $10 of that refundable when you return the key. You can switch parks in the same day without having to rent another cart, but the carts are on an "as available basis" Disney is great about maintaining there carts. I had one die on me in Tommorrowland and they had a replacement for me within 10 min. As far the weekly rental goes i believe it is around $150 from various companies in the Orlando area. The Disablities forums have a great sticky on that