Probably the simplest, safest, most low-tech high intensity interval setup is to alternate fast rope jumping with running up and down stairs or a stationary bike - doing each interval for 30 seconds and allowing 30 seconds rest. The other advantage to alternating interval activities is that fatigue in particular muscles won't hold back your overall intensity. This protocol brings most of your aerobic fitness attributes up very fast and is also the best fat burning exercise protocol... literally almost 10 times better than continuous aerobics. Best to start with 10 intervals total, at moderate intensity, gradually working up to all-out intensity, then add intervals up to 20 minutes. For this style of workout, going past 20 minutes is of little benefit. Further increases come in the form of more intensity.
I just got back from doing a 20 minute HIIT in the park: rotating between running, jumprope, and med-ball situps. Did I mention the euphoric after-buzz? Ahhh..