I like mimi's game!
You could extend it by issuing treats in short intervals, like every 5 or 10seconds, or just take a little bit longer every time you present the treat.
Be sure to up the ante for some of the easy things to wear--or else the propaganda factor might just be too blatant.

You could also have the kids put weird stuff on their heads too--like a bucket or a pillow or a pot, just to see how funny they can be and how different the world sounds.

I also want to encourage you to let her try new helmets at the store. Maybe the old one just hurts or is the wrong shape or something.