I really like leaving a unit test failing at the end of a session. Its much easier to write it at the end than the beginning and they are often pretty easy to complete. Its a surprisingly simple way to get back into things the next time you pick it up and it is I think superior to just leaving a todo list of microtasks because it solves the problem of what precisely you need to reload into your brain to get going, the approach is at least for the first 10 minutes already decided.