Love the idea of letters, diaries etc, wish I'd done that. I have a friend who wrote a piece every month in an album for her first child (dunno if she did it for the second too.)
Each of my children has a year or so of photos in their own album - after that they just go in the family ones. I have coming-home-from-hospital knitted caps for each of them, wristbands, cradle cards and first shoes, and some hair clippings. Draw the line at teeth though!
We have videos going back to when the eldest 3 were 6, 3 and 0 - used to hire a camera then, and just make about 1½ hours of whatever came up. When the third was 2 we got our own camcorder and we do have lots of miscellaneous daily bits which are lovely, also school plays etc. Can we get them turned into DVD does anybody know? (We also have some cine film of DD1 as a baby 21 years ago - no sound - I know that can be turned into VHS but feel that DVD would be safer.)
The other thing I have - not intended as memento but has become one - is family calendars going back 20 years with all the playdates, birthday parties, school activities, out-of-school activities etc.
(Plus all school reports and three huge boxes of artwork which I must sort out some time soon.)