In Pokemon Platinum, you will know that you have to complete the national dex. Well, when you do there is a special quest I think you should know about.
You know that cave that took you to the Pokemon League that time Well you might remember that at one of the doors there was a fat guy blocking the way. Well, now there isn't. So just walk in and a girl called Marley will ask you to help her to get to the other side of the cave.
Now you have a new, fun quest to do!! P.S She has a lvl 59 Arcanine to help you in battles and she will always heal your Pokemon if they get hurt in battle. :)