When I'm about to work on Project Foo, I want to open all documents and spreadsheets related to Project Foo with one click of a link. Is it possible to set up such a link? (Does Google Drive have a concept of a saveable "workspace"?)
No. (Not to say they wouldn't add something like that in the future.)
Of course, if you are syncing Drive files with your hard drive, you could always use your OS to open multiple files using your file explorer (or equivalent).