Whenever I want to work out my abs I have a few exercises I do. I have learned these from my coaches throughout my 2 years of wrestling for my school. Pretty basic but I'm not complaining with the results.

You have your basic sit ups; Feet planted on the ground and coming all the way down and back up.

Crunchies: You legs should form a 90 degree angle in the air, with this you should come all the way up to your knees never fully touching the ground.

Jack Knives: Putting your legs straight in the air while your butt is still touching the ground. Work towards touching your toes then come all the way back down and repeat.

V-Ups: These are a variation of the Jack Knives. Instead of leaving your legs straight up, the will come down with you 6 inches. So you bring your legs straight up while reaching towards then, this time while your back is returning to the ground so do your legs but hold them 6 inches do not drop your legs completely.

6 Inches: Literally as it states just holding your legs up 6 inches. In this you you may add sub work outs to this including: Bicycles, Scissors and Choppies.
Bicycles: I think are pretty self explanatory. While doing the 6 inches act as if you were on a bicycle.
Scissors: While doing the 6 inches cross your feet over and under eachother.
Choppies: While still doing the 6 inches moving your legs up and down never touching the ground and never exceeding 1 foot.

Supermans: Pretty sure this is the exact same as planking. Lay on the ground tighten your tummy, keep your feet off the ground and hold your handing in front of you, as if you were superman!

Push up, Push ups, Push ups!: Not only does this work your abs, but it's obviously great for your arms as well. Don't be limited to traditional push ups. I have a list of different ones in this category too if you want them.

I hope this helped :3