It can be something that you experienced yourself, or one of the many events within the game itself.
Please share

I climbed up the hills east of Bonnie springs at twilight, not far from chances map . It was getting dark, I reached the summit right about the time Vegas' lights started to flicker on. So I just sat there with Veronica and ED-D2 and took in the view.
Killing 5 deathclaws with a single shot from the YCS ( meltdown FTW ! ) is also a favorite.