Take a look at the various types of push ups and add a couple of those in and add some sit ups as well. Get that core engaged. And there are more that one type of sit up as well. Sounds like you are doing some decent cardio with your commute
Well, to be honest, I commute pretty lazily, it's about the level of effort of taking a brisk walk. I have a hard time wanting to work hard on my daily commute (but still it's more excercise than driving!)
I was looking at maybe getting some Perfect Pushup stands (or knockoffs). I'll look into situps, but I find it slightly annoying that situps require 'equipment' to anchor your feet. Probably there are good versions that don't, but the pushup just seems so pure and elemental that you can just do them. (I admire that about running as well, the only problem with running is that I HATE RUNNING)