I'm pretty sentimental I have kept...

the card the hospital writes up for the bed for both boys
all ultrasound photos
pregnancy record book from middies
every single pregnancy test (all dated)
first outfit we put on them after they were born
outfit they wore home
parking meter tickets from when I was in hospital
cord stump as well as the clamp
Moo's hair from first hair cut
all paper stuff from the MCHN
unused newborn nappy
all cards and cards from flower deliveries the boys received
a copy of the birth announcement in the paper it was printed in
all birthday cards
all ticket stubs etc from places we've taken the boys
their first wrap

That's all I can think of off the top of my head. They both have a special box under their bed that it goes into so it's not all over the house. I have done for Moo and plan to for Buster - keep a couple of outfits from each size that I identify with ITMS, eg. Moo's very first pair of jeans etc. As a PP said I would rather keep it for them than throw it out and regret it.