I jump rope - 6 sets of 3 mins with 30 - 60 seconds rest in between. It's not time consuming, it's high intensity, and I can do it on the back porch.
When I'm training for races I run in the evenings after the kids go to bed and it's not so damned hot.
I also read about this workout posted on grapplers guide...
you do 6 mins of exercise with 2 mins rest. 4 sets total...
1st min - pushups
2nd min - cross collar choke abs
3rd min - elbow drags (across the floor)
4th min - bodyweight squats
5th min - side to side bridge
6th min - mountain climbers
30 mins total.