Short answer - whatever you want.
I think chronologically, so I started with pregnancy stuff (ante-natal cards, ultrasound pics etc) -- then did all the birth stuff like announcements, cards, the stuff from his hospital 'bed' thingy, birth certificate, the Medicare card with his name on it -- and the two photos I have of him alive. Anything that showed he was a person, he was real, he was here.
Then I moved on to the rest of it -- I kept everything I could, and I'm slowly getting that all safely scrapped and stored. I plan to include the autopsy report, the death certificate, all the cards and notes I received -- because it all means something to me. I want it all included to show it's importance.
But that is my personal decision. You need to only include what you are comfortable with, and remember that can change at any time and you can add or take things away.
I'll be putting some pages from Levis' album up soon. Like, maybe next week if I can get my act together.