Astronomy apps like Starwalk are amazing. Hold up the iPad to the night sky and see that patch of stars with labels. Really cool.
Theodolite. Hold up the iPad and get an overlay of orientation.
Maps. It is really cool, as a passenger to use the large screen of an iPad to track your travels live.
Note taking. I use ProjectBook. There are many others.
Reading. I prop up the iPad so the weight is not an issue. The point is that the iPad is so portable you can read places where you'd not likely take a laptop and the screen is much larger than that of an iPhone.