I always throw in whatever is in the house.
Trader Joe's prosciutto, Trader Joe's thickly grated cheese, farmer's market sliced cherry tomatoes, and farmer's market heirloom spinach (slightly sauteed before thrown in).
Other delicious omelet concoctions:
-chard (or another bitter green) and sliced onion omelet (make sure to saute the onions a little bit before putting them in)
-mushroom, spinach, tomato, and grated cheese omelet
-sausage and sliced potato omelet (good with an addition of onions as well)
-veggies galore: zucchini, red bell peppers, asparagus, spinach, onions, eggplant, and lots of delicious herbs