It would depend on a number of things. If this is your first half, then I would suggest skipping it.
If you have run several before, and know your body can handle it, by all means do a light workout. I've done several fulls and halves, and have done everything the day before from resting completely, to taking an easy spin class, to doing a more aggressive spin class. When I set my PR, I did a light spin class the day before just to loosen up my body a bit. I purposefully held back in class, but the little bit of action helped my body. The drawback to doing upper body work is having sore muscles during the run. There is nothing worse than having shoulder/back pain from swinging your arms.
When I did an aggressive class, I was training for a full and the race the following day was only a half that I was using as a training run. I still had a great time, but I knew my body could handle it.
The key to whatever you do is light. Dont over do it, especially if you want to set a new PR.