One of the best apps that I have on my iPad is called Theodolite. As you can tell from the screenshot, there is all the information one might need in a very user-friendly interface. Any photographic evidence using this app would be pretty hard to dispute. Theodolite is available in the App Store. It was created by Hunter Research Technologies Inc. and is available for $4.99. Well worth every penny!
SeeLevel is a great free app that serves as an inclinometer or level. It can also be used to measure distances, and is also available in the App Store.
Triangulex is the perfect app for either triangulating the position of an object or place. Simply place the cross-hairs on a distant landmark, then on the object or area in question. It couldn't be easier to use. Triangulex is also a free app available in the App Store.
Of course there are too many apps to cover all of them here, but a search through the App Store will find many tools that will be indispensable to you once you learn to use them.
Besides all the great apps, the iPad does everything that a laptop can do. From Skyping in the field to taking field notes, this is the one tool I don't want to go out in the field without!