We just went through this. My daughter was born in October, a month before my son turned 3. I was pleasantly surprised at how well the transition went. And my sister in law went through this last month with her 2.5 year old daughter.
We did get our son a "big brother" gift. We bought him a toddler camera. Originally, we planned to give it to him at the hospital, but since I was induced, we decided to give it to him the night before. We told him it was his job to help get extra pictures of his new baby sister. We made a big deal out of the fact that it was a big brother gift. Our theory was that while everyone oohed and aahhhed over the baby, he would be distracted by the camera and not get terribly jealous...we knew he would get bored. I think he took pictures of everything but his sister! (the floor, trash cans, my IVs, etc.), but it worked perfectly, he had something to do while he was at the hospital.
His Aunt also filled a little cardboard suitcase with things to occupy him if he is waiting around at the hospital - coloring books, cars, stories etc. I think all the toys were purchased at the dollar store, but he really enjoyed that little suitcase.
As for including your daughter in the breastfeeding...that was a little trickier for me to figure out how to include my son. While we were at the hospital, I just let him sit close and watch. It gave me a chance to explain what was happening. When we got home I started having him help by setting the boppy pillow on my lap. I started feeding her in our recliner, because I learned that he could climb up and sit on the foot rest. Many times I will feed his sister and he climbs up and rests his head on the boppy on the other side. We sit a chat or if he is willing to turn the pages, I'll read to them. And sometimes we just watch some cartoons together. I have really enjoyed that time with my kids.
I made sure to have a step stool near the changing table so he could watch what's going on. That was a great idea. He loves it...he talks and entertains his sister during diaper changes. It also makes it where he can reach the diaper basket. He enjoys bringing me diapers when I ask.
When we brought our daughter home, I knew those first few days would be the toughest on him. I knew I would be consumed with taking care of baby. We tried to prepare him for that before hand - often with books about babies (McDuff and the Baby was most helpful). We had help to entertain him, but I made sure that no matter what - I put him to bed. I made the time to lay with him talk, cuddle and play. A few times that meant that he went to bed later, working around the new baby, but it didn't matter. For that first week I think that time was much more valuably spent together than staying on some sort of schedule.
I was really concerned about the transition before our daughter arrived. I researched and asked questions...I think I over thought things. The transition was rather easy, I think he was as excited about the new addition as we were. Sure, we have had a few rough days...days where big brother was out of sorts and cranky. And days where he wants to be cuddled and carried and tells me to "put down the baby". But overall it has been absolutely wonderful. Congratulations!!!