My good friend, Seth Crooks, showed me a relatively simple little exercise that came to mind when he finished a workout early. This can be done either at the end when you want to work your core some more, or in the beginning as a great warm up.
He used a 35 lb dumbbell, so adjust accordingly to your fitness level.
#1 – Hold the dumbbell vertically in front of you with both hands at chest level.
#2 – Keeping it veritcal, swing your arms in a half circle to the side and down until the dumbbell is now in front of you more at gut level.
#3 – While you circle the dumbbell, also twist to that side at the waist (ie circle the dumbbell left, twist your torso to your left).
#4 – Repeat for the other side
You can do 20 each side for a great warm-up of finish-up. Trust me, if you have appropriately heavy weight, it will feel good (in a my whole core is burning and on fire sort of feel good).