When you get out of Petalburg Woods, you will end up in the north side of Route 104. There are four Trainers to be found here, including your first double-battle in the game.
Flower Shop
The Pretty Petal Flower Shop is a useful place in the game. The first time you visit, you can get a Wailmer Pail. You can use the pail to help grow berries that you planted on the soft soil patches across Hoenn. These soil patches appear at every Berry tree location. Once you pick the Berry, the patch opens up for future planting. A bit later on in the game, this is a good place to get furniture for your Secret Base. There is also a girl who will give you a random Berry once per day.
Buying Plants
After defeating Wattson at the Mauville Gym, you can return to the shop and purchase secret base decorations.
Plant
Price
Red Plant
3000
Pretty Flowers
3000
Big Plant
5000
Tropical Plant
3000
Colorful Plant
5000
Gorgeous Plant
5000
Trainers
Fighting Lady Cindy or Rich Boy Winston will give you a 1,400 reward for an easy battle. He/she has a Level 7 Zigzagoon, which an average Pokémon of level 7 or a few at the average level of 6 will easily take down. Use a strong attack such as Double Kick that can be found at Level 16 when Torchic evolves into Combusken, or, if you didn't take Torchic as your starter, Treeko's Absorb or Mudkip's Water Gun would take this Pokémon out before you know it.