Siggi's + fruit + granola is my go to breakfast
On the road I'll substitute whatever Greek yogurt they have in the lounge (or that I go buy from the CVS / Target / 7-11).
On vacation I love the omelet station so egg white omelet is my go-to plus whatever fresh bread they have (ideally baguette or some other type of crusty bread).
Favorite travel breakfast was in Santorini last May. Stayed at the Astra Suites and they brought breakfast for us every morning (delicious egg white omelet with tomatoes and feta cheese for me) and put it on our outside deck so we were able to eat looking out at the incredible view of Santorini we had from our deck.