You are so sweet to want to make 21 stamped items for your daughter's class!!
I teach first grade and I know how much fun party day can be!! I think the popcorn slider is a great idea. Here's another idea for a box that you might like to try...
http://msrossbec.com/stamp/soft_swirls.shtml
And I've attached a word document with detailed directions from Amy Celona.
You could fill with candy or small erasers or toys. Kids love to open some thing for a surprise (don't we all!).
Or, if you feel like purchasing 21 small fruit cans you could try the pop tops that everyone has been talking about lately...here's a link..
Forums at Splitcoaststampers
LEt us know what you decide! And have fun...
Ashley