Posting this now because time didn't permit in the earlier post.
I try to mix things up when I do H.I.I.T., actually I try to mix things up in general when it comes to exercise.
I go to the local park and high school to use their track because those places are the only 2 places I know that have enough distance for me to run uninterrupted. Sometimes I just run balls out with a regular H.I.I.T. Sometimes I wear a 20 lb vest to switch things up. With the vest on, my speeds are slower, but it keeps things interesting.
Same goes when I lift weight at the gym. Every now and then, 8 weeks on average I'd say, I switch from regular weight/regular reps to high weight/low reps, low weight/high reps or pyramids.
Just be sure you stay hydrated when exercising, especially in the summer. And fuel yourself up properly after you work out.
Another great cardio exercise is plyometrics and as girly as this sounds, spinning.
Oh one last thing, spend about a minute to thoroughly stretch your body before exercising. Stretch from your wrists to your elbows to your arms to your knees to your ankles.