Since this is about five games:
In Pokemon Pearl, I spent most of my time Underground trying to dig for fossils, stones, etc.
In Pokemon Platinum, I spent most of my time in the Battle Frontier because I wanted to get all colored prints (which I still cannot do)
In Pokemon SoulSilver I spent most of my time in either the Battle Frontier and the Pokeathlon. I finally finished the Pokeathlon, but still did not complete the Battle Frontier -____-"