On the streetcar platform in St. Clair West station, there is a pillar which obviously used to have a sign on it, but there are now only the four bolts that used to hold the sign, with a couple of broken sign-bits hanging from two of them. No way of knowing what sort of sign it may have been.
Since the Lynxcub was cranky and out of sorts, I decided to try and distract him by asking him what he thought the sign might have originally said. "I don't know," he replied, not sounding overly interested.
"Maybe it said 'Leash your hippopotamus'?" I suggested.
"No!" he replied, with a grownups-are-silly half-smile.
"How about 'Beware of dinosaurs'?"
"No!" More of a full smile now.
"What about 'All passengers must wear a wombat at at all times'?"
"Noooooo!" We were fully into the my-crazy-muime-is-embarrassing-me mode now, but this meant that he was forgetting to be cranky, so I pressed my advantage.
"This pillar is made of cheese?"
"No!"
"All children under 6 must be riding on a velociraptor?"
"Nooooo!" (extra big grin for that one)
"Airship to Timbuktu docks here?"
"No!"
"No bears on the streetcar?"
"No!"
"Warning: subway may contain antimatter?"
"No!"
Sometime around then, or maybe several more sign-suggestions later, I paused for a bit, not sure if I was entertaining or annoying him. But only a few seconds passed before he tugged at my shirt and said "What else do you think it might have said?" with a little smile of anticipation, so I figured he was definitely having fun.
"All lions and tigers must be kept on a leash?"
"No!"
"Warning: the streetcar wires are not meant to be used as a tightrope?"
"No!"
"Do not eat the tiles?"
"Muime! People can't eat tiles!"
"No salsa dancing on the platform?"
"No!"
Eventually the streetcar had arrived, and as we prepared to get on, I suggested "Warning: streetcar is larger on the inside, and can travel through time and space?"
He gave me a Look that clearly said "I know where you got that idea", and as we boarded, he said "No! It travels on wheels and it's full of ads!"
"So?" I replied. "That doesn't mean it couldn't also be a TARDIS."
At that, a man sitting in the front seat looked up and exclaimed happily "Time And Relative Dimension In Space!", somewhat to the Lynxcub's surprise. So I explained to the Cub that yes, that was what TARDIS stood for, and told the man what we were doing. He listened to the rest of my sign suggestions during the ride with evident amusement and then, just as the Cub and got off at our stop, he said "I think the sign must have said 'Beware of Daleks!'"
To which the Lynxcub replied dismissively "That's silly! Daleks aren't in real life!"
Anyway, the above examples are probably only about a third of the suggestions I came up with - just the ones I can remember offhand. But it was a pretty enjoyable way to kill time, because I got to test my creativity, and the Lynxcub got to not only be entertained, but also feel like he was smarter and more sensible than the adults around him.
ETA (Sep 10, 2012): Almost four months later, and having now moved to an entirely different neighbourhood so we don't see that sign any more, he still regularly asks me to come up with more things it might have said, responding "No, silly!" after each one. And scolding me if I accidentally repeat one I'd already used previously.