Here's one I like to use--you can do it over and over without anyone realizing you're doing it if you just change the details:
The PCs come upon two parties (entities, monsters, groups, group v. monster, etc.) fighting. The PCs are either hidden from the combatants or else the belligerents are so busy that they don't notice the PCs.
The Players' three most obvious choices are: join Party A, join Party B, or just ignore the whole fight and continue on their way.
The funny thing is, the party often acts---or at least my players often act--as if they have no choice at all.
They instinctively join in the fight on the side of whichever combatant looks more human. I think part of the reason might be because most of them are semi-new and don't realize that there's D&D in every direction, no matter what they decide to do.
The situation is like a Rorschach test in a very literal sense--sometimes you'll see an ink blot and it's not just like it looks kinda like something it's like you don't immediately realize that anyone else could see it any other way.
The players, in this way, create both the plot and the morality. Occasionally without knowing it.
Mandy's commentary on one such encounter:
Mandy: "After we fought the guy with the weird brain coming out of his head I was like that was the wrong thing to do."
Zak: "Why was that the wrong thing to do Mandy?"
Mandy: "Well he was outnumbered already even before we got there and then we found out later that the dark elves were performing experiments on everybody. But they were dark elves and Frankie is a dark elf, plus there was the language thing like the brain guy was only making weird bug noises we couldn't understand, but the dark elves could ask for help so we're instinctively like oh, lets help them!"
Utolso Varos, The Last City, and the Mountain to the Moon
59 minutes ago