I'm not sure what your budget is, but I like to include books or craft type stuff, as opposed to cheap toys that will just break and get thrown away.
The Samko & Miko warehouse sale (details are posted in the Family Events & Sales section) have tons of very reasonably priced books. They have lots of stickers, too.
For girls, I include my handmade hair clips/ accessories.
Also, what about food items like home baked cookies or cupcakes?