Just finished the first workout of today. I biked 30 minutes (pretty easy pace and resistance) and then I did this circuit 5 times without a break.
1 minute Mountain Climbers (approx. 40 reps)
1 minute Russian Twist (with 3 kg and did approx. 100 reps)
1 minute Lunges (approx. 30 reps)
I found this workout @ http://hungryandfit.com/2013/01/30/15-minute-high-intensity-workout-no-excuses/ if you are interested to read the whole article!