You usually can't give more than one unit of blood every 2 - 3 months, depending on various factors. You cannot donate if you suspect have any active infection like a cold, have a blood disorder or if you take medication. In many areas, they have blood clinics by the Red Cross or other orgs like that They used to be done by the Canadian Red Cross in Canada but, due to a few HIV infections, they were replaced by the Canadian Blood Services. I don't know about the US.
Sometimes, volunteers help setting up the chairs and equipment and providing refreshments. It's not that hard and, if you help out throughout the day, it can be a good 6-8 hours of volunteering, although most of it is down time which can be used to study. If you give blood on top of it, you get a nice plastic card with the dates you gave blood on it, initialed by one of the blood-draining staff.
You really should consider doing more volunteering, though. It's worth the effort.