cheaply as far as materials-rummage through garages for cardboard, cardboard tubes and such. thrift stores often have partial rolls of foam (or new in packaging foam mattress pads) which can be cut and shaped.
as far as decorating-i can see a simple tweak to a cart to make it into the jungle cruise. give the driver a captains hat, toy megaphone and a cap pistol (maybe put stuffed animals inside and THEY are the riders cruising and seeing the exotic parade crowd). a doom buggy? hit up someone with halloween decorations. some of the car's characters (just the eyes and mouths could pull it off). toy story? sporky would be easily made out of cardboard and spray paint.