When I was there I would cycle through Athena clusters (in my time they were debathena machines and very few people used them), the various libraries (excellent study spaces, especially Barker and Hayden), occasionally rooms in the student center, literally any classroom that didn’t have a locked door (ever done a pset together on a bunch of blackboards? It’s an excellent way to work together), and anywhere you have special access to through UROPs or anything like that if you’re looking for places particularly quiet.

Off campus, the central branch of the Cambridge Public Library or probably most other public libraries (particularly any recently renovated ones)