We have been doing a load of plank holds with our folks...weight on elbows and toes, body rigid. that very simply looking move busts people up. Try a set of Tabatas with that, loads-o-fun.
Regarding "core training" Greg told me it was anything that involved listening to Hank Rollins. it's got nothing to do with abs OR back.
"Survival will be neither to the strongest of the species, nor to the most intelligent, but to those most adaptable to change."