Other than Brown, Caltech and MIT, most schools that have flexible curriculae are going to be LACs. Schools like:
Amherst College
Colgate University
Grinnell College
Harvey Mudd College
Oberlin College
Reed College
Vassar College
Wesleyan University
A couple of research universities have a relatively flexible core as well:
Carnegie Mellon University
University of Rochester
Keep in mind that you cannot avoid writing or the humanities entirely, but the colleges and universities above are very flexible.
Most top research universities with top Physics departments, like Berkeley, Chicago, Columbia, Cornell, Dartmouth, Duke, Harvard, Michigan, Penn, Stanford, UCLA, UCSB, UIUC, Wisconsin-Madison, Yale etc…are going to have some sort of core curriculum that will involve writing and the humanities.