Everything you need for baby and nothing you don't
See also A Word on Toys
Staring is big with infants. They can’t see very well at birth and won’t distinguish colors first thing, so fairly large black and white patterns and pictures are your best early bet for giving them something besides you to look at (not that you’re not gorgeous – of course you are!)
I bought a set of the Wee Gallery Art Cards and stood them up in Astrid’s line of vision in her crib. I have no idea how much she got out of them since she didn’t fill out the feedback card I left in there as well, but I assume they were more interesting to look at than the bars of the crib or the ceiling. The Art Cards are, well, art-y so they won’t drive your adult visual sense nuts and about half of the animals are pretty cute. The other half are scary looking (to me anyway), so I got two sets and weeded out the weird ones. I mean seriously: that fish looks deranged, right?
You can hang the cards from a generic mobile frame, but I never did find one that faced the cards flat down so Astrid could actually see them. Eventually I caved and bought one of ones that hangs the cards vertically, but it was super flimsy and with very little baby tugging it promptly came apart, so unless you’re handy and make your own, I’d skip trying to put them on a mobile.
Another black and white option is the Hoban board books: large generic shapes in black on white or vice versa. Someone gave us this book early on and I thought it was kind of bleak but Astrid took to it eventually and still looks at it, usually in the dim light of her 6AM rising time when the rest of the world is rightfully asleep.