I first learned to take mashed potatoes a step further when I was visiting my friend Katie Skeen in Dublin. The traditional dish colcannon (aka kilkenny) adds kale and scallions to the mashed potatoes. I see this as an excellent starting point.

Note: You don't need a food processor to make mashed potatoes. In fact, they come out better when they are mashed by hand. You'll need a good potato masher and possible a few friends with strong forearms.

Base Ingredients
Potatoes
Kale or Cabbage
Scallions or Onions
Garlic
Butter, Milk, Salt

Extras
(This is where the glory comes in. Choose any of the following or feel free to include your own ideas.)
Mushrooms
Zuchini or Squash
Green, Yellow, Red Peppers

Method
Boil the potatoes till a fork can easily poke into them. You can leave them in the hot water until you are ready to mash everything.
Saute the veggies you want to cook in olive oil: garlic, onions (if you are using scallions, leave them uncooked), any of the extras you want sauteed.
Steam the cabbage or kale, cut into strips or chunks. You can use the potato water for the steaming. 6 to 8 minutes is plenty to steam the cabbage but leave it a little crunchy.
Mash the potatoes in a big bowl, first adding the butter (use your personal judgement on the amount), then the milk to the desired consistency. Salt and pepper to taste.
Mix in the cabbage, sauteed vegatables and raw vegatables.

Leftovers are great as a sandwich with ketchup or fried on a skillet with eggs.