I accidentally found out that mpv can play videos from the TTY, meaning I don’t have to log in to a graphical session. It even has a scaled OSD, unlike anything I’ve seen in a terminal UI.
Obviously there are plenty of terminal applications that work in a TTY, (edit) intext-mode or using ncurses (/edit), but are there any others like mpv that aren’t strictly terminal apps but don’t require a display server (maybe because they do the job themselves)?
Have a look at fbida. It provides fbi (an image viewer), fbpdf (a PDF viewer), and a few other apps that all run in a framebuffer.
There’s also w3m, which is a basic web browser capable of displaying images inline; although this requires some hacking in order to work in a vanilla TTY. What could be easier is a using a kiosk window manager (such as Cage) to launch a lightweight terminal (like st, urxvt, or xterm).