They usually recommend no strenous exercise for several hours after donating. My brother cycled across town on a 100 degree day, donated blood and fainted immediately. He was really down for the count. He still says it was one of the dumber things he has ever done.
I'm a regular blood donor, but I always drive to and from. Why take a risk in traffic when your body has just given away 10-12 percent of it's oxygen carrying ability (red blood cells).
Take care and enjoy.