I've just done some sweet potato, carrot and apple (seperately, not together) Root veg is dirt cheap!
I think the only REAL no-no's are things like banana (goes black) and papaya, but they are easy enough to mush anyway.
You can cook and puree up frozen veg (like peas) and re-freeze, but never defrost and refreeze your meals.
Spices like a touch of cinnamon added to apples makes them a bit more interesting, and you can combine most fruit with fruit and veg with veg.
