If you want to focus on building up core strength, I would think that the basics are still the way to go--and all those variations of crunches don't cost any money. Maybe you can mix up the specific exercises?
The most effective exercises for core strength development are those that involve resisting torsion through the saggital plane, like a simple pallof press or wood/cable chops, not flexion through the frontal plane.
Other great exercises that recruit the full core and posterior chain are heavy squats and dead lifts.
Some isometrics, like planks, are deceptively simple and effective at recruiting the full rectus.
