iPads are excellent devices for students to demonstrate their learning on. There are so many 'power' apps that you, or students, can choose from to use in this way. To demonstrate what they know about a topic students could create a movie, a presentation, a poster, a book, a comic strip, a game and more. If you are unsure of students' abilities you might like to tell them which app you want them to use and work your way through a collection. The idea is to get yo a point where you can ask students to 'show you what they have learned' using the iPads.
One way that works for so many concepts and so many learning areas is having students create a riddle book. The link below is to a very simple book I created about planets of our solar system. The great thing about such a resource is that others students can use it to test their knowledge.
Our Solar System