When making a purchase at a store, you could ask for several "extra" bags and make them your goodie bags.
Many castmembers at registers and sometimes throughout the parks have stickers that they give out to guests, you might be able to collect several of these for free too.
Rather than postcards, it might be nice to have a great photo of your family taken in front of the castle, EPCOT ball, or similar landmark and just have copies of the photo made. I think people who have never been to Disney would like to see you having a good time there.
There are tins full of candy that could be bought for several people (or a classroom) to share. Coffee is quite tasty and is shareable.
Pencils seem to be about $2.50 and up each. Keychains more like $4.00 and up so depends what "cheap" means to you and how many people you're talking about.