#1 would be the phone app, pretty handy for calling people in real time. Without it I would have to carry a phone separately, It’s become pretty hard finding a phone booth today.
#2 Would be SMS, these are quite handy for quick messages without interrupting something important.
#3 GPS maps, I have the most amazing sense of orientation, that basically guarantees I never end up where I’m supposed to, unless I have GPS.
You may take these functions for granted children. But back in the day, maps were something you had in the glove compartment of your car. Phones were something you had on the wall, with a long wire hanging out of them, to connect you to a wired network. Messages was something you send on paper, put a stamp on, and posted through the postal service.
Of course compared to back then, a smartphone has plenty more clever functions. Like the calculator, which we did either in the head or more complex calculations were done on paper.
But I guess these aren’t what you kids were looking for. But they are actually quite amazing, even if you take them for granted. ;)