Raspy: These aren't suggestions for whole cards, but among the kind of info I think would be suitable...

- fishing knots (I tie them so rarely)
- formulae for common water treatments (how many tabs, minutes, etc.)
- watch and shadow methods for finding north

Also, don't forget that these cards may be all the first aid information your buddy or co-survivor has, so some kind of first aid pointers might be worth including.