Tightening your core during many exercises is effective abdominal training. In fact, most bodybuilders don't do core training specifically, they just flex their abdominal muscles while doing cardiovascular routines (i.e. running with your abs flexed).
My favorite abdominal strength training exercise is standing straight with my core flexed and pulling a cable with both hands from an origin beside me.
It's hard to explain in writing, but just note that resisting your abdominals' moving is better exercise than actually moving them (like crunches).