This is another thing that I was really considering - buying my own bokken and then going through some kind of kata by myself at home.
I found repeated shomen cuts to be good. In a broad stance, bending knees as you cut, allowing the tip to hit the ground. That and going into a squat position with the heels touching on a cut. That way I could condition my midsection/hips a bit at the same time. Just go until you can't tense your shoulders. (within reason.)
With kata, or even just four/eight directions, I find it harder to get going. I know they're important, but I just don't reach that shoulder exhaustion that teaches you to relax.