If I were to do sit-ups, I'd use a one-two punch combo so I don't feel like I'm isolating the muscles but that gets boring. When it comes to Core Strength nowadays, I go for the Ab Wheel or the three types I have: The Standard Ab Wheel, The Ab Carver Pro & (the best of them all) The Power Wheel. Each one works the core in various formats but the endgame is conditioning the muscles for long term strength and helping prevent injuries.
The Standard Ab Wheel has been around for decades and is a staple for many athletes that can train their core wherever and whenever. The biggest advocate in my mind when it comes to this device is Bob Backlund without a doubt. A former champion pro wrestler that many arguably say is one of the most conditioned men on the planet and for good reason. This device was one of his foundations for developing into a machine that rarely if ever got tired. Doing hundreds upon hundreds of reps and during his career, would do this for over an hour. It's a simple roll out yet a lot of muscles involved in order to be effective because if you get sloppy, you can hurt yourself.
The Ab Carver Pro is a fairly recent device that was brought out by the people that made the Perfect Push-up products. This has a wider base so you can work more of the muscles along your core including the obliques as you can move to the left and right as well. The handles have a better stance to grip and work your arms as well. It does take some getting use to as it has a steel spring that provides resistance and requires solid smooth motion and control. With a solid nutrition plan and workout regimen, you can shrink your gut and develop stronger abs, not just looking strong, you'll be strong.
The Power Wheel all in all is the ultimate out of the three devices because not only more exercises can be executed, but you have the opportunity to do both upper body and lower body training modalities. Invented by Lifeline Fitness' Jon Hinds, he's taken the wheel to another level and revolutionized the way Core Strength & Conditioning is done. You can do Push-ups, Leg Pull-Ins, Rollouts, Walk On Your Hands, Combos and others, think how strong your Core can be in a matter of minutes a day and getting benefits beyond the standard Rollout.
Get the most out of your Core Training and forget doing Hundreds upon Hundreds of Sit-Ups because in order to effectively train your center that way, you'll have to go to harder exercises and find some form of resistance. With Rollouts and other exercises with the wheels, it'll cut your time down big time and make you work the muscles more efficiently and get them stronger from all sorts of angles without isolating them. Get strong and get conditioned with real Core Training.
