After a bit of research and wading through chatty posts, I have come up with a list. *NOTE: This is a very rough list and there is some speculation as to if you should feed some to your snails or not (you'll see). None of these should harm your fish, but as half of them I have never tried, I'm not making any guarantees, only passing on information in an easy-to-find list. Also, I am talking about the most common apple snail, Pomacea bridgesii here.
Also: all vegetables should be softened in some way before feeding. Cooked, steamed, frozen then heated up (not sure about this one), etc.. but since snails generally eat decomposing food in the wild, they like their food to be slightly softer, easier to eat, and not on the freshest side.
One more thing: I don't know if any research of any kind has been done on this, but I'd recommend organic foods. I mean, I'd rather not take the chance of putting pesticides in my tank for my snails to eat, but that's just me.
Snail Food List
-Spinach (high in calcium, but also thought to "block" calcium from getting to their system to be used, a bit of controversy on spinach)
-Peeled Seedless Grapes (cut in half)
-Cucumbers
-Squash
-Romaine Lettuce
-Celery
-Green beans
-Zucchini spears
-Frozen bloodworms
-Sinking fish foods
-Frozen snow pea pods
-Canned iguana food
-Kale (high in calcium!)
-Turtle pellets
-Corn tortillas (?)
-Algae Wafers
-Baby Carrots
-Snail biscuits (you can make these at home using a recipie, which I've had 2 disasters with, or you can buy them online. There are a couple people who make them, see http://www.pet-bakery.com/Snail/S_index.htm or http://home.mchsi.com/~masts/feed_bag.htm . I am not endorsing, I am just showing you what I mean by snail biscuits!)
-You can always see what is in your fridge and try it out. I'd recommend putting it in a bowl with water in it and a single snail before you put it in the tank so it doesn't cloud your water, mess up params, and kill your fish and snails, plus you can see if the snail likes it or not!
One last thing from chatty little me: snails are individuals. They do not all like the same kind of food; you may have to search before you find one that they like.