alternate between push ups and crunches each night with maybe a rest day, when i haven't got time to go to the gym this really helps to keep in shape. when they get boring/too easy introduce variations, like double crunches, push ups on fists etc. Also really good for a starting point if you've been away from the gym fo awhile and helps you build some basic strength so you don't look so pathetic compared to everyone else! I do about 3 sets of 25 each night.
also really good are ab wheels, you can pick up a decent one for like $10 and are really good for building base strength. hopefully that helps