Check out Rice–a school that is probably not on your radar screen owing to its location near the Texas Gulf Coast. It boasts many of the features you desire (beautiful, pastoral campus; “old” buildings [similar in vibe to those at UCLA, Stanford, and Scripps]; intelligent and inquisitive students) while avoiding many of the pitfalls of some traditionally “preppy” schools (e.g., snobbery; an instrumental, stat-padding, purely pre-professional approach to education; lack of diversity; etc.).
While Rice is located in a major city, its feel is completely different from that of a northern urban campus like Columbia or Penn: It sits in a safe, leafy, tree-lined, and very walkable neighborhood, allowing it to combine the relaxed feel of a suburban or exurban campus with the access to a rich array of cultural activities that comes with a big-city campus.