Ask HN: Fun electronics project made purely out of transistors

I've always been embarrassed by how little I know about practical electronics. I've studied some electrical engineering, I know that I can create NAND gates out of transistors and I can create pretty much anything out of NAND gates.

What are some practical stuff that are digital and I can realistically build soldering together loads of something simple like 2N551 transistors? (I'm not quite sure what are the most common basic types, just picked something that's cheap and seems trivial)

I'd like to create something that's:

1.) feasible (I assume after a certain size there'd be an inherent resistance to the whole thing and I'm not sure how I could manage it)

2.) challenging

3.) type II fun for me

4.) type I fun for others (non-techies can interact with it)

Thanks for any tips!