The Mauville Food Court can be really useful. If you order ramen, you'll be challenged by various trainers with lv65+ Pokemon. Trainers also yield more exp, but unfortunately this can only be done once a day.
Battling the elite four over and over again is a good way to grind for exp, and there is no limit to how many times you can do this per day. You also get a boost for battling a trainer.
You can also battle wild victory road Pokemon, this is unlimited but the Pokemon don't usually give much exp.