For me personally I try to do HIIT on non weight days do I can focus on and get the most out of it and not overtrain on my weight days (that and my time is limited at the gym, lol) My HIIT days kinda look like this:
30-60's (sprint 30 sec normally at 11-12 mph, jog 60 sec normally around 7 mph) for 10 to 15 sets
60-120's (same concept as above) for 5 to 10 sets
I normally finish with some box jumps, chin-ups, planks, and triceps extension leg lifts on the dip bar.