A fairly simple paper-based productivity system, revolving around writing notes and todos as a bulleted list, and regularly reviewing ongoing items, culling low-value tasks. It's a mixture of universal capture and project planning. The system is laid out in The Bullet Journal Method: Track the Past, Order the Present, Design the Future.
Peter Rudin-Burgess suggests using a bullet journal for keeping game notes when playing solo RPGs (Rudin-Burgess, 2023, pp. 14–16).