I've made gingerbread cookies in different shapes with icing piped on. The PTA loved them and sold them for 75cents each. They were all gone an hour later. I have a fantastic recipe if you're interested. All the kids I've ever worked with (ages 3-?) have loved them! Plus, my kids LOVE to help make them.
My brownies are famous...don't know why, really, but I can get any student to do anything if I promise them brownies. It's just the Betty Crocker recipe (from the cookbook) with extra cocoa and chocolate chips. Double for a typical cake pan, and quadruple for a 1/2 sheet pan.
The best seller is Peanut butter chex mix. Evidently (I've sold tons, but never made it), you melt peanut butter and butter, mix it with the chex mix, and coat it with powder sugar. The mom who makes it says it is the EASIEST thing, and she makes it in enormous batches. It sells so fast it's stunning.
Have fun...and good thinking to plan ahead!
Both of these recipes make huge amounts and freeze great.
I love the idea of dipped pretzel rods with sprinkles!