In MK, dd and I always take pics at the wishing well down a side path from the back of the castle (it's not hidden, but a lesser-used area).
There's a store that sells kitchen items, sort of between the entrance to Haunted Mansion and the Columbia Harbor House (right??). I always buy something there and have it shipped home. There's always a kitchen item I could use, and it's a great way to extend the magic after we're home. (actually, I buy little souvenirs a lot and ship them home - pens, stationary, a tote bag, etc). I usually forget what I've bought and surprise myself! I'm easy to please!!
In the Mexican pavillion, there's a a store that sells crystal, etc. They have (or had when we were there), beautiful little jeweled (fake) tiaras. It was small enough to wear everyday (almost the size of a bun-holder), special enough to be a great souvenir, and very girly and fun. We'll both get one our next trip and wear them everyday.
And the Mexico ride - never a long line, totally cheesy, but a must-do for us many times. I don't know why... just a relaxing, cool place to get out of the sun for a while.
I like to pick one country in Epcot and really see and experience every inch of it's pavilion. It's so easy to walk by the little things.
In France, the ice cream cart (maybe they sell crepes there too?) The vanilla ice cream (maybe it's not really "ice cream") is to die for. It's a must have on every trip!
We love PUSH (the talking trash can) in tomorrowland. We must find him every trip and talk to him and get a pic.
When you get off the Norway ride, there are houses/windows in that room. If you stand next to one, you can hear a conversation going on "inside" the house. I can't think of anymore little extra touches like that, but I know there are a lot more all around disney.
It's fun to print out a hidden mickey list, and try to find some wherever you are.