In your Sweet Potatoes Anna with Prunes, what can I substitute for port?

You could also use a fruit juice -- like cider or cranberry juice.

The port is used to rehydrate and flavor the prunes. For rehydration, you can use any warm liquid: even water would be fine, the dish will still taste great! If you want to replicate the flavor profile of port, coffee with a tablespoon or two of brown sugar will get you close to the caramel notes of port.