Super Mario World/Forest Secret Area
A walk in the park level, just stay on the floating platforms and watch out for all of the flying koopas.
If you have cleared all the switch palaces, you can collect other bonuses throughout the level.
At the end, if possible, jump over the finish line to get up to four 1-Up mushrooms. If you have a cape, flying across this level is one of the fastest ways to obtain extra lives. Make sure not to pound the ground on your decent or the 1-Up mushrooms will bounce away!