IMO, the ultimate instrumental break is "Cool" from West Side Story. The tension that creates and the amazing orchestrations, gah!
Other great instrumental breaks include the dance portions of "America" from WSS, the romantic underscore in "Maria" when Tony just keeps speaking/singing her name over the orchestra, the beautiful break in One Hand, One Heart..
And if the dream/nightmare could count as a "break" for Somewhere, that would probably have to surpass even Cool, as it's absolutely amazing how Bernstein transforms the music perfectly, flowing perfectly from the beautiful dream-like music, to the nightmare-rumble music.
There's no pattern among my choices, I swear
Other ones I really like are the dance/instrumental break in "Dance of the Robe" from Aida, the whistling in "Wouldn't It Be Loverly" from My Fair Lady,the break in "Close Every Door" from Joseph, the little break near the end of Music of the Night, the break in "Rent", the dance break in "76 Trombones" from The Music Man, and the brief break in "Memory."
And not sure if it counts as a "break," but I adore the brief clarinet solo in Once On This Island right before "A Part Of Us."
"If there was a Mount Rushmore for Broadway scores, "West Side Story" would be front and center. It snaps, it crackles it pops! It surges with a roar, its energy and sheer life undiminished by the years" - NYPost reviewer Elisabeth Vincentelli