Spend your time with the students by talking with them. Use the board for sketches, mathematical development, etc. This forces you to go slowly enough that students can copy your work, which is basically how people learn. Since you will be talking with the students (with the lights turned on!), you can turn around see whether they are "getting" the tricky parts, and you can adjust easily. None of this works when you're sleepwalking through a slide deck.
This method also works for computing work. Students can learn a lot by watching somebody live-code a problem. Showing a slide full of code might be helpful to a few students in the class, but most will just turn off. Students can learn a lot by seeing a professor type code, building ideas from the inside out, and catching errors, etc.
So that's the mechanical part. On the logistic part, here's something else I've learned: almost anything can be squeezed into the last 20 minutes of a class, and almost anything can be stretched out to fill the last 20 minutes. Students and teachers are humans, after all. Your friend can tell you a story during an elevator ride, or stretch it out over a few drinks in the pub. Either works.