This is a quick ab routine I like to do:
20 sit-ups
20 crunches
20 side-to-side heel touches (2-count: touching each heel once counts as 1)
20 flutter kicks (4-count)
20 Hello Darlings (4-count)
20 Mad Russians (2-count)
20 side crunches left
20 side crunches right
30 sec plank
30 sec side plank left
30 sec side plank right
Run through them start to finish with no rest besides transition time. You will feel the burn. I usually do this at the end of my workout. Right now I workout 5 days a week, so I hit abs 5x week. I usually only do one set, but every now and then I will do two. I'm also a fan of hitting a set here and there of various weighted ab exercises between my main lifts.
The flutter kicks may aggrevate your hipflexors, especially if your not used to them. You can sub those out for a different exercise or cut them out and maybe up the reps for everything else. The point to this routine is multiple exercises with minimal to no rest. These are just the particular exercises I like. You can sub in/out any ab exercises you wish, its up to you. This is a good ab endurance routine for me.