Stonehenge! I even paid a little extra to walk among the stones at sunrise, and in retrospect I should have opted for the couple extra hours of sleep. Stonehenge isn't remotely in the league of nearby Avebury for archaeological significance nor nearby Salisbury Cathedral for impressively stacked large rocks. You can take pictures of it from the motorway; that should be good enough.
Waikiki Beach, after decades of mismanagement, is so eroded that a good amount of the famous white sand is actually imported from Papuhaku Beach on Molokai. There are nice hotels and shops, but I don't go all the way to Hawaii to spend the day in a hotel or a shopping mall. I'd say limit your visit to a half an hour.
The Sears Tower (Willis Tower) observation deck, among observation decks which I admittedly find overrated in general, bears special mention for its costliness. There is a restaurant atop the John Hancock Tower which affords at least as good a view (for instance, you can see the Sears Tower) in nicer surroundings with better refreshments with no admission fee.
The Cathedral of Our Lady of the Angels in Los Angeles is a heap of postmodern wretchedness barely distinguishable from any of the concert halls in the neighborhood. St. Vincent de Paul in Mid-City may be smaller, but "feels" much more like a house of God.