A cup of sugar
I remember the episode in which Maria went into Oscar's can. This is not exact, and I'm not completely sure which was for sugar and which for flour.
Maria went up to Oscar's can and asked to borrow a cup of sugar. Oscar replied that if she wanted it she'd have to come in and get it herself. So she did, and that's where we saw the eyes against the darkness stuff. At one point, Maria fell into Oscar's pool (which Oscar had told us about a few times before. I think he said "Wanna see my swimming pool?" to a visiting grouch that was parked in an adjacent trash can.)
<SPLASH!>
OSCAR: "That's my swimming pool!"
MARIA: "It's sooo COLD!!!"
OSCAR: "Yeah, I keep ice cubes in there..."
Finally they reached the kitchen, where Oscar instructed Maria where to find the sugar.
As the dirty and messy Maria was about to exit the can Luis walked by, apparently reading a Hispanic recipe.
LUIS: "(something in Spanish, then) One cup of flour. Hey Oscar, you wouldn't mind lending me a cup of flour, would ya?"
MARIA: (still shoulder-high in the can) "Don't do it, Luis."
OSCAR: "What do you mean? I can get you that right now." (Disappears into can, and quicly emerges with a cup of flour for Luis)
Needless to say, Maria was a tad P.O.'d at Oscar that she had to delve into the mucky can while Luis got his in a snap.
Maria's entry into the can is corroborated by Wikipedia. See
http://en.wikipedia.org/wiki/Oscar_the_Grouch
I also saw the sketch where Bob went into Oscar's can. My memory of is is a little fainter, since it was about a season earlier then Maria's foray (and I only saw it once). Basically, Bob was walking by the can whistling happily, and Oscar angrily emerged from his can to chew him out for making such a racket. When Bob calmly rebuked Oscar for getting all bent out of shape for mere whistling, Oscar responded with something like "you should see what I have to live with before you pass judgment..." (sounds remarkably similar to the excuse often given for listening to music with violent lyrics and other lifestyle choices) Bob decided to play along and visit the interior of the trash can.
There was another sketch I saw where there wasn't an adult inside the can, but they clearly illustrated the immense interior therein. One day, Oscar announced that he was going to move, and called upon the adults to assist him. We then saw 3 or 4 adults struggling to pull a full-sized hat rack out of the can (Did we see this in Mary Poppins?). The show then broke for a film or two. When they returned to the street scene, the sidewalk was suddenly littered with several pieces of dusty, dingy furniture, all full-size, that were taken out of Oscar's can. At that point, Oscar confesses that he fooled them; he wasn't really moving, he just wanted to re-arrange the furniture in his can. Not taking kindly to being lied to, the adults all promptly dispersed. Oscar said to Slimey "We're going to have a hard time putting all this furniture back.