I visit at off-peak times, but even then the parks can fill up quite a bit from Thursday to Monday. If the park fills up...
MK - TTA, Carousel of Progress, Tiki Room, Hall of Presidents, Riverboat, Tom Sawyer Island, find a quick service to sit outside and people watch. If not crowded, take the boat to Poly or Grand Flo across 7 Seas Lagoon and have lunch or dinner, lots of restaurants to choose, from gourmet dinners to character meals. Visit the monorail resorts by monorail, dine at the Contemporary. Or take the boat launch to Ft. Wilderness or Wilderness lodge and walk around there to decompress. See deer and squirrels walking around Ft. Wilderness, ride the carriage, have dinner at Hoop de Doo review. At Wilderness Lodge there's a geyser that erupts at the top of every hour, kids of all ages (including me) love this little show, some nice restaurants at WL too
Epcot - Energy 45 mins of air-conditioning; sit under the monorail and people watch; ride the monorail to TTC and back again, or just walk around the World Showcase. Some shops are fairly empty when the park is full. Wander back to International Gateway, to the BoardWalk area and walk around the lake there. Visit the 5 Epcot resorts, eat lunch at a restaurant there (huge assortment). There's a 40 minute round trip boat ride to Studios which can be packed on a busy day, but certain times of day is not bad. If boat too crowded I just walk around the BoardWalk area instead.
HS - This is a park to avoid when it's full, lines for anything even Great Movie Ride and Mermaid on a busy day, but there are places to stroll like Streets of America, and the Muppets shop area can be quiet even on a busy day. Take Boat ride to Epcot if you're using Disney transport, or can walk to BoardWalk / Epcot.
AK - Even when this park is full and the attractions are crowded, there are plenty of places to walk and take in the beauty. Discovery Island Trails, little trail by Dinosaur, and the forest trails in Asia and Africa are great places to visit. Flame Tree once you get thru that busy food line, you can sit and relax in the outdoor seating it feels unrushed, just don't feed the birds!