Touring tends to give much more exercise to certain parts of the body than others. Pro cyclists often have unbalanced body development. Some even look like mixtures or chimeras -- half man, half ten-year-old girl. Lower bodies highly developed, strong, fit; upper bodies not.
Are there good ways you've used or come across to balance or complete the workout?
Flexibility, strength, endurance, attitude and energy enhancement are also among the possible results, goals and benefits. The book Spark: the Revolutionary New Science of Exercise and the Brain deals with it from an interesting angle.
It would be nice at times to give the upper body a good, balanced, full workout on tour. Are there good types of light-weight exercise gear that would help with this?
Question.
How long a tour ot months on the road are you asking about.
For most of us, daily life requirements , ie. work, mean that a long tour is a month or less, and an extremely long tour is more than 2 months.
That's important, because your basic upper body condition isn't going to change that much in that time frame, so if you stayed fit before starting, that will carry long enough that there's no concern.
OTOH - folks doing super long tours of a year or so, may have a problem. But even for them, I suspect that the normal activities on tour will keep them fit enough.
So, unless you have a real reason to worry, don't.