I don't work in a donor center but I was a platelet donor for a while at our local ARC. Other than movies or TV to watch while you donated and a snack, there weren't any free gifts or giveaways. I liked doing it because I knew I was doing something important and the people that worked there were all very nice to me. I had to stop going because my veins just wouldn't hold up to being stuck that often. Maybe you should survey your donors to see what they would like or survey the ones that don't return to see if you can find out why.