A disclaimer I'm not a huge souvenir person. Pictures are my main thing- I take ALOT. Anything else I get has to be something that will be used (such as a ball cap) or a toy I know they will play with (DD got the My Little Pony princess dressups this year and plays with them constantly.) Stuffed animals, knickknacks or huge furry hats don't come home with us, lol.
The 2 things we get consistantly (and this is true of everywhere, not just WDW) are a themed photo frame for a family picture (these are all over our living room) and a keychain for our backpack.
My kids do pressed pennies (I think they each did 7 or 8 over the course of the week.) Again, it's something they do for any trip. They're small, easily collected (and stored). Beats 101 assorted junkie trinkets from different places.
As for not buying anything, I've gotten good at "where the heck am I going to put it?", "do I really need another Tshirt?" and "I don't need to be spending money on that to prove I had a nice vacation..."