These kits are SUPER for taking along & keeping your wee ones busy while at a restaurant, doctor's office, or any other appointment where boredom may arise.
I found these awesome plastic containers at the dollar store. Discontinued wallpaper sample books were used to decorate the outside. 
The inside can be filled with all sorts of random objects to keep your kidlets busy: stickers, stamps, paper, notepads, tiny craft kits, crayons, playdough, pencils, pens, small coloring books, matchbox cars, tiny paint sets, etc. The dollar store is a great resource. Also, check out the party favor section of large retail stores for some fun little things to toss in your box! (Tiny paint sets were found at Walmart.)
OTHER IDEAS:
*McDonald's toys are a great size for these kits!
*Have your kiddo help decorate the outside of the box with stickers!
*Create your box with a theme in mind. Pirates, princesses, a miniature Picasso, etc & fill it with like-themed toys & goodies!
*Can also be taken along on car trips!
*Include "wet wipes" for sticky fingers!