Super Mushroom cupcakes are an obvious cake alternative.
Because life is short, I simply covered yellow cupcakes in red fondant . Cupcakes are easy to make, so I made actually made them, using this Smitten Kitchen recipe -- but no one will mind if you just buy them at the store.
The white dots are mini marshmallows cut in half -- I used the gooey side to stick the marshmallows onto the fondant. You could also use green fondant for 1-Up Mushrooms.