Also ask around to your other relatives to see if they have pics of them with your grand parents at various ages, that they may want to contribute. You can always have copies made If they want the originals back.
Most will enjoy being in on it.:thumbsup: Obviously, you will want include them in the credits.
Scrap book sounds good but I would do it yourself. Its an old saying but they will appreciate something you did yourself much more no matter how old you are. Maybe but everything on a DVD slide show. That would be a nice suprise to just start playing at the party or get together or whatever they have.