Have you ever done a hidden Mickey scavenger hunt? I liked the old guide book, because it didn't have pictures, better than the new guide.. It just gave hints and you really had to spend time searching. I suggest that if you can find one, though some of the old HM might now be gone. The AKL lobby has a bunch: in the entrance art, another in the rockwork, another in the designs in the ceiling lights.
In MK, find all of Pascal's Friends hidden in the garden near the Rapunzel bathroom.
After that, the HM gift shop now has some unique items. If you have not taken time to explore the longer HM queue, it is a bit of fun, especially at night when the lines are short, and you get the area mostly to yourself. Play along with the mysterious ghost writer for a bit.
There's also a game you can play with the little crabs in the LM queue. You help them sort Ariel's thingamabobs from the junk by pointing at them.
The Pixar short films (Epcot) are fun to see, and there are fun water fountains outside the theater. Best of all, you will likely have them to yourselves.
Shopping might not be your thing, but the Disney Springs Marketplace Co-Op is worth some time to see unique items that aren't sold elsewhere.
Have you been to the (relatively) new-ish Trader Sam's tiki bar in the Poly? It is only open certain hours.
Other posters mentioned some of the hidden spots in the World Showcase, but there are more. Japan, Norway, Morocco all have little hidden museums and nooks.
Another person mentioned one thing to do at Ft Wilderness, but not everything. An offbeat thing to do at Ft W is to tour it in December when the folks staying there put up rather elaborate personal decorations, but it is worth exploring year-round.
Like you, we often try to get to MK as quickly as possible, but the WDW shuttle boats, are another way to tour the area at a slower pace. Consider the ones inside Epcot and the MK area boats. For that matter, if you have never rented a WDW watercraft, that can also be fun!
You may well want to visit the new CSR! dining.
Another small way to vary WDW visits is to pay it foreword and see what happens. It can be anything: offer 1st timers your better parade viewing spot or your bus seat. Here in 2019, kindness can sometimes be seen as creepy, so you have to be careful, but even in 2019, it is sometimes possible to offer unexpected kindnesses to strangers. We often buy bottled water for WDW, and at the end of the trip, we give away extras. I'm always amazed at how much strangers appreciate a free bottle of water.
If you haven't explored Universal recently....