In the NE, along with Swarthmore, have to add Williams, particularly broad and deep in math/science.
If you're willing to look in the Midwest, Carleton has some of the strongest physical science departments among the nation's LACs. Economics is strong as well with its own great study abroad opportunities in Cambridge and Beijing (70% of the campus participates in some form of off campus study): Carleton College: Economics in Cambridge Carleton College: Political Economy - Beijing: Program Description
I'd also suggest considering Grinnell and Oberlin - both present strong offerings in the sciences.
If you'd consider venturing all the way to the West Coast, Pomona will offer a well balanced program across the board. Reed will be strong as well, particularly in the sciences, but offers a more niche experience.