I substituted sliced rhubarb (from our garden!) for the sliced apple. Instead of the diced apple in the batter, I put in one very ripe banana, and since I only had one egg, so I added another very ripe banana (did you know you can substitute banana for egg in a recipe? I didn't, before I Googled it!).
I cooked the butter, sugar and rhubarb a bit too long and the rhubarb got a bit too carmelized, but it didn't taste burned, it just looked too dark.
And the proof of the pudding (or the cake) was in the tasting!
It was delicious!
I love it when an experiment turns out right!