It's more of a concept book than specifically opposites, but I've always really liked
Inside Outside Upside Down. I think your mother gave it to us. It was written by the Berenstains back before the dreaded Bears (I'm so not a BB fan), and it's wonderfully engaging and simple. It has to do with a bear getting into a box, the box being loaded upside down onto a truck, and then bouncing off the truck and landing right side up. The bear runs home to report, "Mama mama I went to town: inside, outside, upside down!" A cautionary note: this comes in a board book and a jacketless hardcover. Don't get the board book: most of the Bright & Early Board Books series have been abridged. I don't know if this one is, but the others are fairly drastic. The cover has been tarted up to make it look like a Berenstain Bears book, but inside it's not.
I've always been a little ambivalent about
Food for Thought: all those vegetable people feel a little weird. That said, though, I do like
How Are You Peeling?: Foods with Moods, which isn't quite as busy at Food for Thought. One of several good books on emotions.
Rachel posted a comment looking for good poetry books for the 2-to-5 crowd, which I fully intend to speak to, but I want to rummage around in the shelves at the store before coming up with a list of favorites. Where does Eleanor stand on poetry?