After getting your second badge in Sapphire, a good idea would be to teach a Pokemon HM Flash (obtained from a Hiker in the cave north of the town) and search for Steven Stone in the same cave. Next go to Slateport City via Mr. Briney and deliver the Devon Goods (I think that's what it's called)
Tips: Catching an Aron could prove useful considering it's a steel type.
An Escape Rope would be useful in the cave but I think there's one in there.
Chat with our AI personalities
the third badge in sapphire is at mauville city(north of slateport city)
In Pokekon Sapphire, you get the 5th badge after defeating your opponent in a Gym Battle at Petalburgh.
in lavarage town
The fifth badge is in Sootopolis City, you need Surf, Dive, and Whirlpool to get it
You get in right before getting the last badge.