Marvelous Bridge is one of the many new locations to explore after you complete the main game. To get there, just go to Nimbasa City, then head east to Route 16, and go east again to get to the bridge.
Buy a Magikarp
In Unova, you can't find Magikarp all over the place. In fact, you can only get Magikarp in one location in the entire Unova region, and that is right here on Marvelous Bridge. A Magikarp salesman will sell you one for 500 Pokédollars. Not only is this the only way to get a Magikarp in the game, but it is also the only way to get Gyarados.
If you have gone to Village Bridge and spoken to the woman who has four Patrats, she and her Patrats will be here at Marvelous Bridge. She will play Patrat Shuffle with you. The objective is to guess which Patrat is holding a Big Mushroom. If you guess correctly, you get the Big Mushroom. If you don't guess correctly, you lose the Big Mushroom but you can keep guessing until you find the Patrat who has the mushroom. You can play this game once per day.
Orbs from the Shadow Triad
The first time that you go to Marvelous Bridge, the Shadow Triad will show up and give you the Adamant Orb, Lustrous Orb, and Griseous Orb. These items boost the attacks of Dialka, Palkia, and Giratina.
Ace Trainer Glinda
There is one trainer here: Ace Trainer Glinda. She has an L65 Alomomola, Klinklang, and Beheeyem.