Since I have Platinum, I always seem to be in the Survival Area battling the daily Gym Leaders that show up at the Battleground. o3o Although my Pokémon's levels are already way beyond 60+, beating the Gym Leaders still provide a buttload of experience points, so it's worth it. ♥
And the place I visit second most is probably Jubilife City, 'coz I'm determined to get at least second place in the lottery. >:D I always end up winning the fourth prize, which is just a free Ultra Ball (how that can even be CONSIDERED a prize, I'll never know).