Rather than tell a child in advance what's about to be seen, it can be wonderful to let them feel they've discovered something on their own. A surprise can be so stimulating that the memories will be more vivid. And the discovery becomes a personal accomplishment.
If the parent is surprised too, that's a bonus, but if you can allow for someone else to be surprised, it can be a kindness and a gift.
photo by Lydia Koltai