One of my favorite less than 5 minute math activities on the iPad is to give students a number and ask them to show me 4 ways to get that number. At the beginning of the year it is usually addition, subtraction, a picture, and tally marks. As we progress through the year students get more creative, combining multi-step problems and multiplication or division.
I usually ask them to do a Number Talk once a week. Any drawing app would work for this. I want students to keep it simple, though, and not spend a lot of time on the bells and whistles, so I use a web clip of www.chalk.37signals.com. There's white chalk, pink chalk, an eraser and that's it.