My Script Calculator, a free app, is a unique calculator that interprets the mathematical expression you write returning a calculated answer.
One teacher is having students use this for multiplication fast practice up to 10 x 10, where student A writes a problem and student B must provide the answer before the calculator returns the solution. They keep tally back and forth to see who can get the most right. Note, you need to be fast returning your answer as the iPad will be rather quick.
Check out the video to see some examples, but most of all try, experiment, and have fun.