This app is free until April, though know that some content becomes an in-app purchase. Bookabi allows students to create their own storybooks with 2D and 3D characters. They can upload their own photos and pictures plus save and share their final projects.
Use the app to create and manipulate 3D objects. They can then be printed out as flat nets and turned into physical objects. This is fantastic for teaching elementary students spatial skills.
Though an iPhone app, this would also work on an iPad. Star Walk is a very cool app that essentially is a mobile planetarium. Find visible stars and planets overhead and take the classroom outside.