You mentioned you wanted something for off-days? So you want something that'll maintain what you've built up in your real core work, but not so much that it starts interfering with the new mass and tone formation that's the result of that real work (remembering that muscle cannot build at the exact same time that it's being overloaded). Also, you want something that leaves you plenty of time for other muscles you're really working on "off days" for your core.

I'm an old guy, so I have an old-fashioned suggestion that might fit the bill: timed V-sits. Start one your back with only your glutes touching the floor, shoulders and feet about a handspan off the floor. Slowly raise both feet and shoulders until your forward-reaching hands can touch your feet, and then slowly back down to handspan-off-the-floor. The first time, time it so that it takes five seconds to go from near-straight to V-sit, and then five seconds back down to handspan. The next one even slower: ten seconds up, ten down. Then fifteen seconds up and down, then twenty. Twenty-five. Thirty. Then reduce the times in five-second increments until you're back at five seconds to get to V, five down.

If this isn't enough, start at thirty-second timings and work your way up to a full minute. Add slow twist-Vs for the obliques. Don't let your feet or shoulders touch the floor at all during this, not unless you're interested in possible long-term lower back problems.