[JohnFen]

> Some topics seem too small or obvious to be worth half an hour

You might be surprised. The most common mistake I see in presentations is that their topic isn't nearly narrow enough, so my advice is usually to present one small aspect of the larger subject. I bet you'll find that even something that seems trivial or obvious can easily fill a half hour with worthwhile discussion.

Maybe pick something small, and if you don't fill the time with the presentation, have a Q&A session afterwords. That'll soak up all the time you have left.