Redbook magazine has some great ideas for substituting veggies in recipes.
Take a look here
You can also add a can of black beans to any box of brownie mix.
Rinse the beans then return to the can and add enough water to fill the can. Puree and add to brownie mix (don't add anything else). It doesn't smell that great and looks horrible, but once the brownies are cooked they are delicious.