What foods are good pickled
Vegetables like green beans, beets, cucumbers, and green tomatoes.
Being British, I am very partial to pickled onions.
Don't forget fruits!!! As they can be awesome!!! Pickled peaches (when in season), pears, and even strawberries (think balsamic vinegar) can be pickled. Peaches are my number one recommendation though.
Brussels spouts, aka Frog Balls! Pickle them as you would spicy pickles. They make for some amazing Bloody Marys, or are great on their own!
Anything is great pickled - everything mentioned already and also consider cauliflower, peppers, garlic, cherries, ginger...the list is endless.
If you are looking for ideas, check some Turkish, Moroccan and other Middle Eastern cookbooks. Pickled vegetables are a great favorite in those countries and walking through outdoor markets, you see huge displays of all sorts of interesting pickled foods. Think preserved lemons, cabbage, beets, okra, beans, . . . .
Meat also can be pickled. Has been for centuries.
Eggs can be pickled. Can't think of what cannot be. Milk, butter?