I'm a huge fan of donating anything one can, I think it's really the closest you get to a true miracle. I wasn't allowed to donate blood for many years because of the extremely strict guidelines here (I'd spent just over a year in Africa as a child, fancy that). When they loosened up the guidelines a few years ago I signed up, and have now donated blood maybe 8 times or so.

I feel fine afterwards, but I have low blood pressure and wooze easily so I drink before, during and after. I'm fine with gentle activity the same day and the next days, but nothing strenuous, and that can last for more than a week unfortunately. In February I went for a long, but not fast, xc ski trip 4 days after donating. I felt exhausted, got a terrible cold that turned into bronchitis and didn't kick it until 3 weeks later. I might have gotten ill anyway, of course, but I think I was pushing it a little. I'm more careful now and try to time donating to periods when I can take it easy for a week or so, avoiding energetic exercise. Relaxed bike riding and low key weight training, no problem.

I know other people who say they have no reaction at all, so you just have to try. It's great otherwise