Try kicking a shell into a block that contains an item just as you scroll that block into the screen. Depending on the level, the block could change to blue, purple, or brown.

You can even change a shrub into a green block in the auto-scrolling World 3-6. Jump on the first Koopa Troopa, then kick his shell to the right a few seconds after the next platform appears on the screen. The shell should be repeatedly hitting the multi-coin block on the lower platform as the next shrub scrolls in from the right side, which will turn the shrub into a green block in the background.