To be fair that's the thing I definitely would want to avoid. "Having to figure out the event". Not just because that's unfair. But also because of how volatile it makes the whole thing. Just imagine what would that lake even be if one of the players didn't prepare a "breathing underwater magic"?! What if no one did??!! What if none of them opened the egg and was like "what are we supposed to do here in front of this lake?"
That would be a very awkward day.