Justin has recorded an awesome follow-along core workout that requires no weights and just twenty minutes.
Full list of exercises:
Middle-Left-Middle-Right
10 reps
Right Knee on Left
10 reps
Left Knee on Right
10 reps
Right Leg Straight
10 reps
Left Leg Straight
10 reps
Modified V-Sits
10 reps
Knee Tuck + Russian Twist
10 reps
Side Crunch (both sides)
15 reps
Penguins
12 reps
6 Inches
~70 seconds