One of my students' most favorite activities with the iPads uses a QR (Quick Response) app. I have tables in my room, and on each table I place 2-4 math problems numbered. Students then have a document that allows space for work for each problem. Students move around either independently or in pairs at their own pace solving the problems on the sheets of paper. 
In order for them to move to another table, they have to search around the room for their matching number. The matching number has a QR barcode for students to scan with their iPads that gives them the correct answer for each problem. Once they scan the matching problem they just worked on and have the correct answer, they then can search for an empty seat at another table and continue working through the problems. If they are incorrect, they then have to go back to their original table and make corrections. 
I usually have some type of competition where students earn some type of award if they are the first couple groups to finish. There are tons of sites where you can create these QR Code readers. I often use QRstuff to generate QR codes for the answers of the problems. One of the lessons I recently used is about exponents and is free here.
