Expo buses go to and from resorts and expo. You could use the Contemporary Resort expo bus while at Magic Kingdom, with a short walk or monorail hop.
Swan or Dolphin while at Studios, Beach and Yacht, out the back of EPCOT.
You don't need to return on the bus that got you to the Expo. It may be a bit of a challenge finding another resort bus stop at the expo. You are looking for MEARS Expo buses, usually yellow in color.