Want a simple way to do Interval or HiiT training? The benefits to this are easy to attain. The work is short and hard. You expend as much energy as you can in a mere 20 seconds, then rest for 30 - 60 and do it again. A total of 3 times. That is about 3-5 minutes work.
SO if you have no stationary bike, no rowing machine, etc. To do this on...merely drop to the floor and do pushups. Do 20 seconds worth as many as you can, then rest and do it again.
No equipment needed.
Can't do pushups? Start with a few only, exerting the strength and energy is what this is about.
If you only get a few in 20 seconds that is ok. You will get up to 20 or more in no time.
Pullups are great also. Deep squats with out weights work too.
Get the idea?
Now go for it.