I know you posted this awhile ago, but since there were no responses...
If you haven't already answered your own question, maybe I can help. What do you mean by 'light-off-roading". Are you talking about gravel roads, fire trails, and single-track, or something like light mountain biking? For things like gravel roads, fire trails, and single track - you could consider a cyclocross bike. All the same geometries as a true road bike, but with wider stays to allow for beefier, knobby tires (up to 35c). By nature, I"m a strict roadie, but I do like cyclocross in the rain and mud (as well as for gravel races).
I'm a fan of different bikes for different purposes, and don't switch tires/wheels for drastically different scenarios (i.e. don't use slicks on my 'cross bike - cross is for 'cross, the roadie is for the road). So in short - if you can, I would recommend your city bike for "city things" and a second bike for "dirty things".