Do you know any good ab stretches, I never seem to know how to “cool down” after an ab workout???

Put your hands together like you're praying and press them against each other. Depending upon how you shift your weight, you can feel this stretch anywhere from your arms to your abs to your shoulders. Another one is to lay flat on your back, point your toes, and lift your arms over your head until you are flat like an arrow.

Honestly I have no idea lol. I usually lie on my back and tuck my knees and pull them closer to my chest. Other than that I do a general cool down. For example the yoga cat pose, cow pose, baby pose and such.

Stretch your arm over your head to the opposite side of the body while putting your other hand on the ground next to you and making sure to stay up by pushing on the ground with that hand. Stretches the sides of your abs.
Cobra yoga pose and cat cow yoga pose too.

I would recommend some side stretches and maybe a downward dog to an upward dog. I would also recommend looking up videos on YouTube for a really good cool down