Sky Map is one of the oldest AR apps out there. It was originally developed by Google. You turn it on and the phone displays a star map. You hold your phone up to the sky. The display will show the stars in the sky right above it.
You can use it to find constellations, It’s a little tough to calibrate at first, but it’s reasonably accurate once you figure it out. It’s fun for entertainment, especially when you’re enjoying the great outdoors. It’s totally free with no in-app purchases.