Rides, character photos, picture of all the group in front of the park icons (MK-Castle/AK-Tree of Life/MGM-Sorcerer's Hat/Epcot-Spaceship Earth -- note: these are usually on the first page of each park section), eating (and photos of food), parades, funny photos (usually staged -- going into shops and trying on hats especially).
I normally make my book up chronologically - Day 1 (when we leave -- either by plane or car), then each park in order (once again, with a group photo in front of each park icon to start off the group) and what we did in the park(s), our hotel room, pool, trip home. I pick up any freebies I can at the parks (napkins, popcorn boxes, stickers -- ask cashiers in the shops for these, swizzle sticks, etc.), buy postcards, get autographs from characters, etc.