I also really liked the GIANT Coloring and Activity Pads because they were about an inch thick with coloring pages, but they are the half size coloring books that would fit really well on the bottom of a shoebox without having to be bent.
There were LOTS of other items that can be included in the shoeboxes! Since everything is $1.00 at Dollar Tree, you can go wild in there and not spend much :) Some of my other Dollar Tree favorites include the small stuffed animals, small footballs and bouncy balls, and slinkys. You can also generally find a small selection of tools, and sewing and craft supplies that make great additions to the 10-14 boxes.
Dollar Tree is a great place to take the kids with a shoebox and let them buy items to fill their shoebox or let them buy a grocery bag full of items to donate for the boxes for packing day. Take this list of Suggested Items with you if you are not sure what to get!