I've always wondered if you could do something with dot-matrix-on-aluminum-foil. It wouldn't look the prettiest and the information density would be low, but it seems like it would be cheap and last forever.
What format should I invest in (developing/purchasing/etc) that would A) be easy-ish to read from B) be dense enough to exceed the point I might as well just be using paper/glass slides/microfiche/and so on, and could be ramped back up from pretty crude tech.
Combine "end of civilization", "time-travelling with tech", and other thought-experiment scenarios for fun.
Don't worry about advanced stuff. How to make a transistor will be lost long before we develop a new civilization capable of needing transistors. So don't waste paper on it.