Paper Mario (N64)
Somewhere in Boo's Mansion is a vase about as tall as Mario. Jump into it from the nearby crate, and Mario will jump back out as a paper Mario from the 8-bit Donkey Kong games! Though the graphics resemble Mario from Donkey Kong, you're treated to music and sound effects from Super Mario Bros. Jump back in the vase or leave the room to return to normal.
Pleasant Path (between Toad Town and Koopa Village) is the home of a few blue, striped plants(?) growing near the background. All of them have eyes, but only one plant's eyes will follow Mario as he moves around. Each time Mario walks a circle around the plant with moving eyes, a Heart will pop out of it.
When you see the chapter title screen as you start a new chapter, stay on the screen for about five seconds to hear the original SMB main theme play.