It’s hard to beat sprinting for a good HIIT workout but what happens if you don’t have enough space? Try T-Sprints
Arrange some markers in a T shape with each line approx 10-15m in length (or just work with whatever space you have)
Accelerate from marker 1 to marker 2, side step from marker 2 to marker 3, side step from marker 3 to marker 4, side step from marker 4 to marker 2, run backwards to marker 1
Repeat for 1-3 rounds before resting and repeating.
The constant accelerations, decelerations and multidirectional running will really get the heart racing!
If you want to add an extra element throw in some calisthenics like push ups or burpees each time you get back to the starting position.