ecnarwold......I bought cute summer outfits from my favorite stores. I personally love rompers so I got the terry cloth ones from baby Gap and others from Gymboree. A onesie and shorts works too. I really liked the ones where you could buy a sweatshirt or sweater too, they come in handy if you're out later at night, if you go somewhere air conditioned, or to make the outfit last into the fall a bit.
A Heads Up for anyone who is finding out the gender, usually at the beginning of January Gymboree runs a big red balloon sale and has lots of items $6.99 and under. I got full summer outfits (hat, romper, socks, accessories) for $3.99 and under per piece. Usually they have tons in newborn and 0-3 so even if you're waiting to find out, it's pretty reasonable to buy one of each!
As for other things not mentioned, you might want a swimsuit to take cute pictures in :). Also, depending on your lifestyle and summer plans you may want a sun block that is safe for infants. Big floppy hats to cover baby's bald head while outside are nice too.