Hi, Nik - -
You might try simple push-ups, and at first you may want to do them with hands and knees on the ground (instead of hands and toes, which you can work up to). Note that this will focus on the upper back, not the lower. They're boring, but you can do them anywhere, any time...I find it helps me when I decrease my excuses to skip exercise ("can't get to the gym", etc.). Good luck!