Pilot16 - I think the carb ice detector sounds a great idea. It's exactly the sort of real-world problem which your average teacher would love - a bit exotic, but not so complex that it takes more than a couple of paragraphs to explain the concept.
I believe most carb ice detectors simply measure the temperature within the carb, so adding the humidity gauge may even be something new. The most complex bit may be calibrating the two inputs (ie temp / humidity) so that you can predict the formation of ice. You may not need to actually do that bit, so long as you can come up with a viable scheme for carrying it out that you can write up.
I'd suggest this one to your teacher - bet they think it's a good idea.