I had one of those the other day during math. I've had the free Skitch app on my iPad for quite a while but I haven't used it in class. I also have my iPad hooked up to an Apple TV and projector in my classroom which allows me to mirror my iPad screen through the projector.
I was about to recreate a graph from our textbook on the board when it dawned on me that I could snap a quick pic of it through the Skitch app.
I can project web images, photos, screenshots, or blank pages (which would work like a whiteboard app) onto the screen and then use handy tools to annotate the document.
Here's the pic I took from my teacher book. This is exactly what the class saw on the projector.
The kids were engaged, paying attention, and participating! I'm so thankful this little app popped into mind at the right time.