What could be a good substitute for the plums--which are hard to come by for me? Banana?

Can you get prunes? They're dried plums. You could soak them or just snip up pieces of the dried fruit.

Apple slices, maybe?

Banana seems a little off for me. I would say any stone fruit would do and I could see pineapple giving you that "upside down" vibe and then I might a little brown sugar in the bottom.