Things that weren't in the film adaptations, either due to technology not being available to show it on film, budget, or cutting due to run time.
Little detail: in the Philosopher's Stone, one of the ways the Hogwarts letters are delivered to the Dursley house is rolled up inside a dozen eggs, passed through the window to Aunt Petunia (as the door is already nailed shut). I would love to see Aunt Petunia cracking open eggs and finding a Hogwarts letter inside each one.
Big detail: The whole scene in which the Weasley family attempt to come get Harry from Privet Drive ahead of the Quidditch World Cup, by connecting their fireplace to the Floo network, then can't get out as it's an electric fire, end up blasting half the living room apart and Fred & George "accidentally" let Dudley eat a Ton Tongue Toffee.
What's yours? One big detail and one little detail allowed each!