Do you ride any hills on your commute? When you stand and ride aggressively, you work your arms. Any time you're riding, actually, you can make a point to flex your arm muscles and strengthen them.

I actually do this periodically throughout the day as I think about it. Sometimes while driving, sometimes while watching TV or just walking from one side of the house to the other (I don't work). I just tense up my muscles and hold it for a while. Rest and do it again.

Sometimes I push my hands together in front of me with my elbow sticking out, or just move my arms in different ways to work different muscles.

This only works the arms (and maybe the shoulders a bit), but it's something your can do almost any time, anywhere.