Teaching is a very good way to learn and remain sharp in my opinion. I have been fortunate enough to have been a teaching assistant for a multitude of courses during my time as a graduate student at UMD. Some of those course are:
ENME480: Introduction to Robotics
ENME361: Vibration, Controls & Optimization I
ENME462: Vibration, Controls & Optimization II
ENME350: Electronics and Instrumentation I (voted as the best TA for the semester by the students)
ENME489Y: Remote Sensing
ENME662: Linear Vibrations (Graduate-Level Course)