This is the best workout I've found for outside the gym - The 7 Tiger Moves. Here they are:
It looks easy. It is not easy and in fact is some of the most difficult exercise I've ever done.
In addition, you should check out Super Health by KC Craichy (the guy talking in the video). In the interest of full disclosure, he's a bit of a crazy Christian, but there is nothing offensive in the book, and his nutritional and exercise science stuff is sound. The book has photos of all the 7 Tiger Moves and clear explanations.
These are martial arts training moves, which use only muscle tension and contraction to build strength. When you're at home and have access to a gym, it's good to bring these into your rotation, too.
And of course, most ab exercises can be done outside the gym.