Where will you be staying? That might help us tell you what the best transportation options are.
Assuming you will be relying on buses......I'd plan to visit the Boardwalk on a day you are at either Epcot or Hollywood Studios. Very easy access to the Boardwalk from either park. From Epcot exit the International Gateway, then head left over the bridge to the Boardwalk. From Hollywood Studios I'd recommend the boat. From the Studios get off at the first stop at the Swan and Dolphin. Turn left off the dock, then another left and over the bridge to the entrance to the Boardwalk promenade. You can also walk from the Studios to the Boardwalk...about a 12 minute walk.
While at the boardwalk you can shop (Screen Door, candy store) and eat (numerous places, all types of food from junk to hot dogs to pizza to fancy sit down), grab a few drinks (Big River for beer, Belle Vue Lounge inside the Boardwalk Resort for full bar, ESPN Club if you like sports, Jellyrolls for the dueling piano bar, Atlantic Dance Hall if you want it loud and dancy), sit in the Boardwalk lobby, take in the entertainment they have on the Boardwalk in the evening, rent a surry bike, play carnival games (the lobsters are addictive, and the only carny game in all of Disney we do....sort of a tradition for us). Plenty to keep you busy for a couple of hours.....or longer, depending on how much you eat and drink while you are there.
Finish up by Hollywood Studios closing time, which will give you plenty of time to get back over to the Studios to catch a bus back to your resort.