Dates, times, addresses, phone numbers, frequent flyer info, confirmation numbers, hotel, rental car, and other contact info.
I also do all the math to understand at what time I should leave home or the hotel to be at the airport on time.
If I have to rent a car I'll map the route from airport to hotel and print a Google map with directions.
I do the same for any meeting, visit or location during my stay wherever I am going.
I'll usually send that document to my wife if travelling alone or, if it is a family trip I'll send it to a family member so they have all our trip info handy (you never know).
I'll print one copy for each person travelling. That goes in your bag, wallet, pocket, whatever.
And that's it. As I said in a prior comment, I have yet to be at the airport and thought "I should find an app for my iPhone". Our travelling is usually very relaxed because all the information we need is in one place.
And it does not require power. Maybe that's the key. As a long time sailor I learn the hard way that anything electronic can and will fail exactly when you need it. It can also be stolen or damaged. I don't want to find myself in a situation where all of my information lives behind a small screen that I just lost or can't power-up. Paper still works wonders...you can even write notes on it.
That being said, as a developer I don't see any reason to be anything other than excited about this. I think you've been pretty lucky to have never run into an issue while traveling. I've had my fair share of run-ins with last minute gate changes, too-tight layovers, incorrect signage at overcrowded airports, etc. If I could build my own little Twilio integration that alerted me the second a gate change occurred, that'd be awesome (and super fun to build).
Then again, we're living in the golden age of integrations right now and I get overly excited about all of this shit : )