The Tower of London is the not-to-miss-at-all-costs historical sight. It's so rich in history, glory, dark secrets, death, that it's well worth a day's visit.
Plus, it hosts the Crown Jewels (my lady loved those).
Do not miss the Yeomen's (Beefeater's) guided tour, those ceremonial guards are an attraction in their own right. Humorous and very descriptive, their narration made me laugh on several occasions, while learning a lot of the Tower's history.