Most of central Florida, including WDW, is extremely flat.
There are a few hills that I can think of off-hand -
* Entrance to MK - all of the park is build about 15' higher than the surrounding grade to allow the Utilidors to be above the water table, so there is a slope up from the Seven Seas Lagoon to the park gates. It's gentle enough to push a chair or stroller up it without too much trouble.
* England and International Gateway at Epcot. France is a high point, so you have to go up a hill in England to get to the bridge - and back down again if you take the turn out to International Gateway.
* Arched bridge from Yacht and Beach clubs to Boardwalk. The bridge is built high to allow for boat traffic below.
* Pleasure Island - the West End Stage is a good 10'-12' higher than the rest of the island, so the main street is a gentle slope and there are stairs and elevators behind the stage leading to the West Side.
As Cannot Wait 4 Disney posted above, the monorail stations are all on a second-story level, accessible at MK, TTC, and Epcot by means of ramps, and via the 2nd floor lobbies at the 3 resorts (actually the 4th floor at Contemporary).
These hills are all isolated and not terribly steep, so you won't have any trouble dealing with them on your trip.