We bought a Chicco Caddy for our trip this past Dec. Our son is 4, weighs 41 lbs, and just made the 40 inch height requirement for Splash Mountain.
The stroller cost around $40-50 at Babies are Us. It is very lightweight aluminum, folds into a caddy bag, similar to the nylon camp chairs, and had a shoulder strap to carry it by. It also had a canopy, reclined a small bit by undoing a zipper on the back (it was enough for our son to settle in for a good nap!), and came with a RAIN COVER!!!! It came in very handy a few cold wet days we had, for sure. We added a mesh bag to the back, large enough for an insulated lunch bag full of juice boxes and peanut butter crackers and our rain ponchos, a sweatshirt, and a change of clothes for our son.
Stroller rentals are about $8 per day, I think. Our 7 day trip would have cost $56 in rentals. So we actually saved, and were able to wheel in the sleepy boy from the bus to the hotel room each night. Oh, and our son does not nap at home, but always does in the parks, so it was a great place for him to have quiet time and rest, or sit for the parade, etc. It was totally worth it.
We are buying a second one for our younger son, so we don't have to lug the full size stroller for the next trip!