You could use it as follow-up practice on new language presented in class. Many of the older course books (Streamlines, Grapevine, the Strategies series) have ready made cassettes for drills. You could also use it for pronunciation practice, the Headway Pronunciation series has useful materials for this.
Either the entire class could do the same exercise or you could allow the students to work in self-access choosing the ones they find interesting or have difficulties with.
The lang lab provides excellent practice for anyone who needs to improve their skills on the telephone. You probably have a way of connecting two booths to each other. If there's a technical person in your school, they'll probably know how to do it. By connecting 2 booths, students can speak to each other and get used to listening without visual clues (eg hand gestures).
If you connect 2 booths, you can also use the lab for pairwork practice. You could use any pairwork activities you would normally do in class. The beauty being that it's very difficult to peak or cheat.
As fillers at the end of the lesson, you could do a mini dictation. You can read out large numbers that they write down in figures or a little spelling test.
Hope this is useful