During the all too short riding season, I don't do much more than routine maintenance - chain adjusting, checking pressures - those kind of things. I do seem to spend an awful lot of time telling people what my 1973 Turbo'd Z1 is as the tank has no name on it, and the side covers just say "Turbo".
In the winter, I often will work on bikes just to learn about them. Each year I try and buy a different bike as a learning experience. In the past few years that's iincluded Suzuki GS750's, Honda CB750's (SOHC & DOHC), a Yamaha Virago 1100, Suzuki GS1100 shaft, Kawasaki KV75 and a few more. I ride them for a while the next year to understand the bike better before selling it off.