[bambax]

Here are two not present in the comments so far.

Print subtitles (useful for old TVs that can't select a subtitle from streams or files; I use this to get my kids to watch movies in English):

    -vf "ass=subtitle.ass"

or with .srt and in a huge yellow font

    -vf "subtitles=subtitles.srt:force_style='Fontsize=36,PrimaryColour=&H0000FFFF'"

Extract 1 second of video every 90 seconds (if you have very long footage of a trip from a dashcam and you don't know what to do with it, that makes for a much shorter "souvenir"):

    -vf "select='lt(mod(t,90),1)',setpts=N/FRAME_RATE/TB" -af "aselect='lt(mod(t,90),1)',asetpts=N/SR/TB"