Your local city college may or may not offer free classes (in San Francisco, you just need to show proof that you live in the city with some legal status).
Some public transportation is free for certain groups (youth and folks experiencing homelessness can get free passes here).
"First X of the month" at the zoo/a museum/whatever
lots of venues have free events.
A jog, bike ride, hike
lots of great stuff outside!
A lot of non-graphical utilities
basically the *NIX coreutils, plus stuff like rsync, ssh, compression/archival tools (tar, gzip, bzip2, etc.), grep, and the like. Git also comes to mind.
I think part of this is that the UNIX philosophy is "developer friendly"
tell a good dev they need to make a compression utility that follows this protocol, and they will make a compression utility that follows the protocol.