"The bright and shiny city of fun." is a good way to describe Mauville City, that is if you like riding your bike back and forth ad infinitum and throwing your money away at the Game Corner (maybe I was just bad at it). What's that? No bike? Well you actually can get two technically at Rydel's Bike Shop, the Acro and Mach. Head over to the gym for a fight against Wally in front of his uncle. You might be impressed this sick young lad made it all the way here from Petalburg, but he actually has family located not too far out west from here himself.
The gambling center gives the player access to the big three of Ice Beam, Thunderbolt, and Flamethrower if you accumulate enough coins. You first have to have picked up some Harbor Mail from Slateport to get a coin case from a young lady in Mauville before you can do that though. Luckily that bike of yours gives you total access to the suspended bike path looming over Route 110 for a quick ride.
The game corner has both slots and a roulette wheel game. I'm gonna be honest, I have little insight as to how to excel at these games. Instead I offer to you the theme to listen to in consolation:
https://m.youtube.com/watch?v=9oeiQ9Gtg1QI like to head west for some experience before taking on the gym. Wattson's a toughie so we might need it!