I am just looking to mix it up on the treadmill, Right now I do 50 minutes running at 8.5mph at 2% incline and then 15 minutes of HIIT 1 minute at 13mph and 1 minute at 5mph alternating at 2% incline, I was just wondering if people could mention some other ways of doing HIIT that I could try out for fun, just to stimulate myself really. Much appreciated and I can't wait to try out some your suggestions.
You could likely see greater improvements by moving your HIIT workout to before your long run, and by shortening your HIIT intervals.
Moving the HIIT training to before the longer run will allow you to attack it with a greater intensity, which works well, as intensity is the key component of the HIIT strategy. A 50 minute run at 8.5mph is tiring for the vast majority of people, and afterwards it will be difficult to fully commit to sprints. Moving the longer run to the end will also serve as a good cool-down for your body.
Shortening your HIIT intervals will also allow you to increase your intensity, as your can run faster for a shorter period of time. Start off with something like 20 seconds on, 40 seconds off x 10 and aim for a pace of 15mph or so (if your treadmill doesn't go this high, you may want to consider moving to a track). If this feels easy, try bringing it up to 30 seconds on, 30 seconds off x 10, or try increasing your speed. Keeping a shorter interval will allow you to push harder on each effort, and will maximize your benefits from HIIT.
If you do decide to make the switch to an outdoor track, it will be far easier to divide the intervals by distance than by time. Instead of 30 seconds, you could run for 200 meters; you may also find it's easier to push yourself when running for distance, as "the faster you run, the quicker you're done".
Maybe try a steep incline for some of the high intensity intervals. I find that ~5 mph at 15% incline is quite intense by 2 minutes, but if it is not enough for you, pick up the speed.
Another tough one is stepping (slightly hopping) sideways on an incline. I do 15% with a 3.4 mph speed for one minute per side, resting in between. Again, pick up speed if you need more intensity. Be careful with tripping (takes some coordination).
Also, walking up incline backwards. Harder to get intensity for heart here, though, but good leg burn.
Are you really doing a 4.6 min mile for 1 minute? That's impressive!