Pokémon Ruby and Sapphire/Rusturf Tunnel

Not much here, except Whismur, which provides 1 Hp EV point, and with the low levels it allows it to be a great EV training spot for Hp.

Also...
When you first come in here, you will fight the aqua grunt that kidnapped Mr. Briney's Wingull

Aqua Grunt

Also, later on you will need to use Rock Smash (HM 06) to complete the path, and you will receive HM 04 (Strength).

There is a hiker you also battle on reaching the rocks to smash.

And, when you take the entrance from Ventdurf town, there is a path to the left leading to outside where you can find blackglasses as an invisible item on the ground.

Items

 * Poke Ball
 * Blackglasses