ColAR Mix is an Augmented Reality app which is free but you will need to download and print the "trigger" colouring pages. There are a couple of free ones but the others cost $2.99, unless you are lucky enough to grab them when they are a featured free app :-)
Students colour the page then view it via the ColAR app on their iPads. An animated 3D model will come to life from the page.
The dragon scene is very popular, but the students also like designing their own sneakers and seeing them in 3D
Watch a video tutorial here - ColAR