One year for Christmas I gave everybody Cake in a Jar. (Actually I think one person got brownie mix, but still) It was a pretty cool hit. Instant cake! Who doesn't love that?
The basic recipe is cake mix plus a complimentary pudding mix. Once you've got that, any cake is made up of a cup of that mix, an egg, one tablespoon of oil and one tablespoon of water.
That much batter will make about two five inch cakes, or four cupcakes.
There's two ways to cook that - two minutes in the microwave or 20 minutes in the oven. I like the oven method better.
Making it portable and entirely fuss free? I found Deb-el dried eggs, and put the equivalent measure of egg powder in the mix (then add more water when the time comes to cook).
As for oil, I haven't tried hard to find a substitute, but I think dried lethicin granules might do it. Mainly the advantage of the powdered eggs is that I can let the kids lick the bowl without worrying about salmonella.
We've had a lot of fun with differently shaped cupcake cups. "Can we have star cakes, Mom?"