You can always find something to do. Take the time to explore areas you haven't visited before.
Examples:
• Route 115 North - An assortment of secret bases. A couple of trainers too.
• Route 123 East - You have to go south from Mt.Pyre, but you get the Solarbeam TM and battle trainers. Bring a Pokemon with Cut!
• Route 120 - you pass through it, but most people are in a rush to get to Lilycove so they miss many things on it.
• Route 124 (sea) - there are places to dive that lead to shards.
If that gets boring, then you can always keep raising your Pokemon, growing berries, filling your Pokedex, ...
There are contests and the Battle tower (in Emerald the whole frontier), there's the Trick Master if you've forgotten him. You could chuck all your ubers out of the team and all your EV- SUPER- *insert whatever adjective could be used to describe it* -trained Pokemon that have been trained precisely, bred, with everything calculated and the correct nature and movesets, and get a Taillow level 3, a Ralts lv 4 and (let's say) Cyndaquil lv 5 and return to Littleroot Town. Now go around Hoenn again, training until you go crazy.
And if all fails, there's the obvious "Select + ^ + B" method which erases your saved game and you start the game from scratch.
Ultimately, there's your local games store where you could buy a version of Pokemon you don't have. And once you've strained every drop of fun from your games, it's time to face the great outdoors. Go hiking, take a trip into nature or read a good book. Or just wait for the next installment of the Pokemon games. It's up to you.
