My son was 3.5 when his sister was born. Honestly, those firs few months are such a blur of sleep depravity that I don't remember much. I DO remember reading him books while I nursed, cuddling and dozing through a Sesame Street episode while he was fascinated. Small stuff like that. If he wanted to help take diapers to the garbage we let him, if he didn't we didn't push. 95% of his involvement with his sister was by his choice (or not).
We also made sure to do special things just for him. One was swimming lessons. They were something HE could do that the baby couldn't. And, since we were going to a YMCA I could check her into the on-site daycare for 90 minutes, which gave me just enough time for his lessons, getting him showered and changed then we went to get her.
They're now almost 8.5 and almost 5. They drive each other crazy, like most siblings do, but neither one will let anyone but THEM hurt the other.