I get a lot of ideas and inspiration from the World Wide Web. For example, this is totally awesome. Because, you know, only a kid would eat a cookie that looks like a petri dish of growing bacteria.
On a similar note: this is pretty cool as well. Jezebel - celebrity gossip AND classroom activity ideas, all in one place!
I think I would have these pre-made and then have younger students identify the parts before eating it. Older students can frost the thing themselves - with some plans drawn out, submitted to me, before they even touch the piping bag.