8 minute is all you need to get a good abs workout and this one based around variations of planks and holds. Let me know how you do in the comments.

40 seconds work | 20 seconds rest

Elbow plank
Side plank (R)
Side plank (L)
Plank shoulder taps
Slow mountain climbers
Lateral plank walk
Elbow plank knees to elbows
Bicycle crunches

source