We have found that the traditional method of setting aside blocks of time for students to drop in for questions or discussion has not worked well. Instead, you might consider telling students that you can be available before or after class, or on an as-needed basis, by appointment.
Many instructors often use spaces in the ExCollege building to meet with students. Other popular locations include the Kindlevan Cafe in the Science and Engineering Complex, the Commons or Hotung Café in the Campus Center, and the Tower Café in Tisch Library for student meetings.