Again, a secret that I shouldn't remember, and did. On 3-1, you can drop down into the waterfall, and there's a secret room down there. When you go inside, you can get a ton of coins in the negative zone. 
I really like the world bosses in Mario 2. They're pretty interesting. I like the mouse who throws bombs at you the most, I think.

That makes me think, though. Maybe it's not that everything is really big in the Mushroom kingdom... maybe Mario somehow got shrunk down tiny? Think about the number of things that are ridiculously huge in the Mushroom kingdom. Mushrooms, for example. Mice, turtles, pipes... maybe they're all normal sized, and Mario is actually the tiny one?

So far, I've only used Mario. Maybe I should change it up, and play as Toad or Luigi? Princess sucked, imo, so I probably won't use her.