My younger brother is taking his girlfriend out for a belated Valentine's Day dinner this weekend and asked me for a recommendation. They are both 21 and live around Porter Square. Here are the constraints: T-friendly, more fun than fancy, entrees around $20 max (they are both poor college students), and they are not particularly adventurous when it comes to food. I would normally recommend Addis Red Sea as a fun and inexpensive place, but I don't think they would like it. . .so something more American. Maybe Cambridge One? Thanks!