From the Super Mario Wiki
The player starts off on a platform next to some Toads, Hammer Bros. and Boomerang Bros. At the end of the platforms is a Warp Box that leads an area that contains several platforms along with some Goombas and several other enemies. At the end of this section is a Warp Pipe that leads to a boss fight with Boom Boom and Pom Pom (with Pom Pom's boss theme used). After defeating them, a Warp Pipe appears.
In the next area is a Switchboard that goes past some Fuzzies, some Tail Bob-ombs and some Fire Bars before leading to a Warp Box. In the next area are several Donut Lifts and Fire Bars with a Warp Box at the end. In the next segment is a Big Cosmic Clone, some Burners and some Chain Chomps with two Wallops guarding the Warp Box.