1646 Mass Ave.
Cambridge, MA 02138
daily: 0700-2100 (deli hours; store stays open until 2300)
T: Harvard or Porter (pretty much halfway between)
Montrose Spa is an old-fashioned corner grocery with a deli. It just happens that they do a great Cuban sandwich: pork, ham, mortadella, swiss, pickles, and mustard, on french bread, pressed flat on their grill. The sandwiches are pretty good value for $5.95.
They do a variety of other sandwiches, none of which I've tried yet.
In nice weather, you can sit at one of the tables outside.