I was going to recommend signing time dvds but you already thought of that! Sam loves the signing time videos too, I think she has a thing for Alex. It was probably around 17 months when I realized she was yelling out his name when they sing the intro song.

At least 2 1/2 hours isn't toooo bad. Hopefully you will be seated near other kids/kid lovers because that in itself would work to entertain Sam and it seems most babies/kids love to watch other babies/kids. Adults who love kids are good substitutes for other kids.

Sam won't sleep in the car anymore either unless she's really tired and it's dark. I thing after a point they are too aware of what is going on and don't want to miss out. It's a tough call - do you try to make Hannah tired so she does sleep or would that backfire and make an overtired cranky baby?

Since Sam is now 19mo old it's easy to remember what entertained her at 17mo and unfortunately for us crayons were way too early, still are, but she is very into picture books that are just pictures of objects that she can identify (I think she is happy to show off knowledge of things) and she likes to play with containers or buckles. I'd say pay close attention to what entertains Hannah at the time and pack that as carry on. Make a list to help remind yourself of all the tricks to try - healthy snacks, pat-a-cake or similar games/songs - if you're nervous about the flight you might panic and forget your ideas!! (I know I would, my husband is the calm one.)

Good luck and have fun!!!