Try this "Buck Fifty" workout.
I've no car to get to the gym and I don't have enough time to walk in. It's also pishing down here (no surprise -- November in Northern Ireland, like) and I don't want to go out in it. Sad fact, but true.
Still, I did three sets of this Big Brandon Carter ab workout in around fifteen minutes (that includes how long it took me to roll out my wife's yoga mat) and I actually made a wee "whoop!" noise when I was finished. The dog looked confused.
By the way, I've let life interfere with my workout time over the last three weeks, so I managed this without being at the top of my game. My other challenge for the day will be to do 100 press-ups, 100 squats and 10 chin-ups (yes, that's ten, not 100 -- my shoulder is still letting me down).
I'll be fitting these in after I get some more work done. If I can squeeze in a trip to the gym when I get my car back, all the better. If not, that'll do. For today.