Andrew Welch's Bomb Shelter goes on just about every Mac I've got.

I also put the Programmer's Key INIT on every Mac I've got that is missing a programmer's switch.

For all of my System 6 Macs, I install TrueType. I also include the SaintFrancis font, which is a TrueType public domain font that is essentially San Francisco.

Did I mention Teach Text? Oh wait, it comes with every installation of the OS.