After some brainstorming (and some not very good-looking samples you don't want to hear about) we decided on shape and then cut the box out of a cute gift wrap paper she had purchased.
They turned out nice enough and I decided to scan one of the non-assembled boxes, create a template and post about it here in case anyone wants to do something similar.
The link to download the template is at the bottom of this post and I suggest you print it directly on lightweight cardboard (since we originally used wrap paper we had to glue it on cardboard prior to cutting, but trust me, it takes a lot longer).
All you need to do is print the template...
...cut around the edges...
...fold it along the dotted lines...
...and add glue to the two side flaps to assemble the box:
And now you have one small favor box. Bear in mind that it is a small box, so only candy or little trinkets will fit in there. Just to give you a better idea of size, here it is in the palm of my hand:
My daughter decided she wanted one to store her ponytail holders (though she needs more like twenty boxes instead of one) and another for the Barbie shoes (those do fit well in there), which proves that the little boxes can be used for other things besides party favors.
Anyway, here is the template in case you want to download it.