Other ideas
Full courses
There could also be continuous series of lectures about one subject - i.e. a full course. For example, course of Theory of Computation. Taught probably just by students and not free (because otherwise no one would take it seriously). But for try-out it could be integrated to the standard meet-ups.
Lecture movie nights
Organize MIT or other lecture movie nights - gather together and just watch non stop Quantum Physics lectures or some other stuff.
Chemistry and electronics lab
It would be great to have a very minimalistic chemistry lab to do some basic experiments for educational purposes. Ant it would be also nice to have electronics lab - all in one place: components, soldering, etc.
Book of Everything
Geometry textbook for school
A textbook only about geometry at roughly highschool level. For Lithuanian schools. But probably would be great idea to write it also in English. And, ofc, have some digital version of it for visualizations, automations etc. The content would begin with very strict axioms and everything would be very rigorous. Also would include a geometry tikz tool for drawing geometric figures. Maybe also using Coq Proof Assistant.
Other axiomatic systems
Strictly formalize other axiomatic systems, somtehing like Principia Matematica. Or just analyze more the Principia Matematica and then rewrite everything in humanly comprehensible language.
Philosophy
The final product would be a philosophical system that would describe how the whole world works, very similar to Bertrand Russell philosophy, but more formalized. In other words - the whole book would be about matematical logic and mathematical philosophy implementation in general philosophy and everyday life.