I always thought that the Battle Frontier in Emerald and Platinum were really good additions for post-story gameplay. They focus on battling, which the player should be more than adept with at that point having beaten the Elite Four, and it gives you the opportunity to collect the symbols or whatever they're called.
Platinum's Frontier was perfect. They offered online multiplayer so you could partner up with a friend and take on the different areas together. It was really fun, but they took that feature out in BW's Subway, which sucks. :\