I can also recommend bulk roasted unsalted peanuts. They have some advantages: they're cheap, they don't crumble, so you can always have a few in your coat pocket, and they take some work from the crows. They'll enjoy the challenge, and you'll enjoy watching.

I once lived in territory of some ravens, and it was a deep pleasure to gradually become friends with one. It took maybe a year of consistent effort, but eventually he'd come hang out with me outdoors. I would say bits of Poe's "The Raven" to him, and when his turn came he'd respond with soft, friendly mutterings. He especially liked to visit when we'd grill and eat outdoors, as there was nothing he liked better than some some bone with bits of meat and gristle left on it.