+1 for rowing, it's great for the lower back which is where i beleive many people have problems related to deconditioning.

not to mention the benefits to your cycling that come from 1) combining an aerobic exercise with core conditiong and 2) that it does great things for your flexibility and position on the bike. just look at the position of a rower at the top part of the catch, rotate it, and you have the aerodynamic tuck of a good cyclist.