Mauville city there is a restraunt you get tons of money for it daily
The easiest one is the Village Sub Combo, which costs 1000 P, and you battle L17 Pokemon from 5 trainers. Each battle gives around 1000 P at the end (if you have Amulet Coin equipped). If you manage to complete the battles in 5 turns, you will get a Nugget. If you manage to beat all 5 trainers in 5 turns, you will get a Nugget and a Figy, Aguav, Mago, Wiki or Iapapa Berry.
The second one is the Magnemite Croquette, which costs 5000 P per order, and you battle L55 Pokemon from 6 trainers in double battle format. This is the most lucrative one battle prize money wise, as there's a Rich Boy and a Lady who will give you about 5000 P each (10k if you have the Amulet coin). If you complete the battles in 6 turns, you will get a Pearl String. If you manage to beat all 6 trainers in 6 turns, you will get a Pearl String and a Metal Powder (which is useless...). Note that you must have a fire/ rock/ ghost/ dark/ flying move as the final trainer uses 2 Shedinja.
The final one is the Mauville Ramen Bowl, which costs 10k to order, and you battle L65 Pokemon from 8 trainers in triple battle format (but the last trainer uses L75 Pokemon). If you complete the battles in 8 turns, you will 2 Big Nuggets. If you manage to beat all 8 trainers in 8 turns, you will get the Big Nuggets and a Deep Sea Tooth (OR)/ Deep Sea Scale (AS). This is the hardest one to complete, as all 8 trainers have combinations on their Pokemon which cause you to lose turns to paralysis/ confusing/ flinch etc.
If you sell the Nugget, Pearl String and Big Nugget x2, you will get a total of 32,500 (5k, 7.5k, 10k e/a).
Here's a link to the Serebii article that lists all the trainers & their Pokemon.
If you want experience, you could level grind and battle 36 Blissey at level 100 each in various secret bases. Every Blissey gives about 19000-20000 each, and I've posted the links below.
Secret meadow: https://plus.google.com/106404913615929340538/posts/LFq9YA2BHFw
Secret shore: https://plus.google.com/106404913615929340538/posts/J9SoQfM2vyf
EDIT: There's also a smogon thread where many users have created Blissey Bases so you can have about a dozen of them in your game.
Note that bases will not work if you have your own base in the same location or you already have a base in that location.
Hope this helped :D