Play the casual game Tic-tac-toe without a gameboard to improve your short-term memory. Sure, one could play it without an app - But now, you can stop arguing with your friends about who did a mistake. IMHO, this is superior to methods like Dual N-Back for improving your short-term memory - because it's fun! I have done this, without an app, for the last few months and I can already see an improvement in my memory.