Reveal to son!

So as my partners mum has wanted to be a grandmother for sooo long my partner is 33 yrs old and she is 60 her wish is finally true and she comes over to visit us in 4 weeks we have decided to tell her when she is here by making up a box with stuff in it so we havnt told our 6 yr old (my son not my partners) and will tell him at the same time as my mother in law I decided to start a box for him today too any other ideas what to put in it ??