My experience [ as a UK Masters 100m/220m sprinter ] for HIIT - at our club, on the track , one very effective HIIT protocol is : Racing start, sprint all -out 25m, decelerate [ which typically lasts about 3 seconds ], turn round jog back SLOWLY towards the 25m mark, role-in to another 25m max effort sprint, turn round, repeat for a total of 8 runs.
So you have sprinted 200m , but the near-constant acceleration/decel is very energy sapping, so , if timed by an observer, your 25m time will drop off by around 7-10 per cent by rep 8 - the performance drop-off is a good thing, just maintain the very high intensity. Walk 400m slowly, and do again, number of sets according to your fitness level.
Or, continuous 400m laps, sprinting for 10 secs, then jogging for 60 secs, etc is very popular,completing 2-3 laps......otherwise hill sprints are good - I think that for other exercise types, the key is finding an exercise that involves very large numbers of muscle groups - so very high heart rates including a real challenge for the working muscles will be effective, whether it involves strongman type exercises or anything else. Experiment with an open mind, discover what you enjoy and what works for you - and attack it with a ruthless and complete determination, the effort is certainly justified for the potential results.
Good luck !