As far as I am aware, a wheelchair (available for rent at the parks and from a number of local companies) would be the only option. The outside companies tend to be cheaper, and renting from one of them means you'd be able to use the chair everywhere you go, a useful option since many of the longest walks will be to and from park entrances (or hotel rooms) to transportation. Segweys, etc. are not allowed, and I think you have to be 18 to operate a motorized scooter.
When it comes to touring, obviously, you'll want to take it easy, schedule lots of breaks, and use in-park transportation options (e.g., Friendship Boats in Epcot between Future World and World Showcase, the Walt Disney World Railroad around the Magic Kingdom, etc.) to the extent it will save steps. Visiting during times of moderate weather and lower crowds, and staying in a preferred room near a transportation hub, at a compact resort (e.g., POFQ) or a monorail resort, can also help.