Here are a few HIIT workouts I like to do:
#1
3 minute warmup at 3.5
15 minutes of all out sprints 30 seconds at level 12.0/60 seconds rest (shown in this video)
2 minute cool down at 3.5
20 minutes total

#2
3 minute warmup at level 3.5 (no incline)
15 minutes of thirty second sprints/30 second rest at level 10 or 10.5 (depending on how I feel that day)
For the last 5 minutes I do thirty second sprints/thirty second rests at level 11, 11, 11.5, 11.5, 12
2 minute cool down at level 3.5
25 minutes total

#3
3 minute warmup at level 3.5
5 minutes of 30 second sprints at level 10.5/30 seconds rest
5 minutes continuous at speed 3.5 and incline 15
Repeat the sprint and hill part
2 minute cool down at level 3.5 no incline
25 minutes total