I usually just put a timer on for 10-15 minutes and do any exercise that pops into my head for strengthening the core. I'll usually do all abs except for one back, hips, or glute exercise in the middle to break it up. I find that this is mentally a lot easier than counting reps or having structure. I am taking my break after track so I do this every day but when I'm running I try to get it in three times a week after runs in addition to spontaneously doing a few while on my phone or petting the dog or whatever when I'm bored. A few of my favorites are:
Planks/Side Planks (Sometimes "Thread the Needle")/Forearm Planks/etc.