I've used: a group shot in the airport/with the car on a couple and on one book, a picture that we purchased in the park on the front page.
What to look for: I bring home at least 3 copies of each park map and anything free I can get my hands on - paper napkins, copies of recipes from the restaurants, if you get iced tea in a sit down restaurant - it usually has a pic through the lemon from the park you're in, ask for free stickers in the shops. I've also taken home popcorn boxes and I have asked in the counter service restaurants for basket liners (looks like tissue paper to me), French fry baskets and more. No - I'm not shy in the least. I do try to explain that I'm a scrapbooker and I've never had a refusal from a CM (yet!).
Have a wonderful time!