Pretty much any veggie, unadulterated is good. (Not corn.)
Also, fruits are good. Frozen berries are awesome (blueberries anyway, larger berries are harder to enjoy frozen).
Basically fresh-esh plants are generally good because they have a lot of water and fiber to fill you up with.
I'm also a big lean pocket fan, though it doesn't compare on the fill:calorie ratio with the above.
Oh, oatmeal is also great (again, unadulterated).