At AK, we love looking at all of the carvings in the roots of the Tree of Life -- walk on the trails to get right up close to them. At Epcot, we always have to try the sodas at Club Cool. At MK, it used to be Cinderella's wishing well on the right side of the castle but I'm not sure if it's still there with all of the new construction over there. A second choice for MK would then be that wet marble spinning globe thing in Tomorrowland near the entrance to the Peoplemover -- my boys love pushing it around. Not in the parks, but on the way there, they love to ask for transportation cards -- they're trying to collect the entire set!
Edited to add: I almost forgot I have to add a second one for MK -- we love trying to catch one of the old vehicles up Main Street first thing in the morning. We've ridden the trolley and the double decker bus. Still trying to get the fire truck some day!