My baby's almost 3 months old and here are a few things that I have found most useful so far:
SLEEPSACK: Eventhough you are having a summer baby, you will want this once the cooler weather hits. Its a safe way to keep baby warm and is easy to put on. my little one uses it every night!
PLAY GYM: At 11 weeks, my son started playing with his toys and I have found this to be the most useful. He can lay on his back and reach for and grab the toys. Sometimes he'll lay there for an hour or more and play, before falling asleep! The one I have is the Gymini Super Deluxe Light and Music (Babies R US), which has plenty of toys for tummytime too.
SLEEP POSITIONER: This was probably more for mommy than baby, just because it gave me peace of mind about SIDS.
MONITOR: The one I have is the Fisher-Price Dual Sounds N Lights Monitor (Babies R US). I think it's great, eventhough its one of the cheapest ones you can get. This is because it has only 2 channels. So, if you live in an area where you might get a lot of interference (like if your neighbors also have babies) you might want one with more channels. The one I have will light up when sound is detected, which is nice if you are doing something noisy, like watching TV or showering.
HEAD SUPPORT: My carseat came with one but it wasn't very good. I registered for the Velboa Snuzzler (babies R Us) which gives head and body support and is super soft and cozy.
DIAPERS: At least register for some NB and size 1 diapers... the last thing you want to do when you have your baby is have to stop on the way home from the hospital to buy them! I loved the Pampers Swaddlers for my newborn. They seemed to fit a tiny new baby better than Huggies did. (mine was born 6 lbs, 12 oz)And, Pampers are much softer than Huggies.
BOUNCY SEAT: We keep ours in the bathroom and I put the baby in it while I shower. That way I can still hear him and I know he's okay. I've been doing this since he was just a few weeks old and he's done great!
HEALTH AND GROOMING KIT: I have the American Red Cross 17-pc. Health and Grooming Kit (Target) and it has everything I need from a hairbrush to a thermometer.
MOBILE: I didn't realize how much my little one would like to stare at his mobile! I just got the one that matched his bedding set but I wish I would've invested in a more expensive one. Mine is just wind up, which is annoying because I have to go in the nursery and rewind it. I would suggest you splurge on something battery operated, like the Symphony-in-Motion Mobile (babies R Us).
PLAYARD: If you have a 2 story home, this is especially helpful. I have mine set up all the time in my family room. It has a changing table built in so we don't have to go upstairs everytime we need to change a diaper. Its really nice for the first couple of months when your baby is sleeping a lot too, because you can have him/her in the same room as you... so you can stare at and marvel over how amazing your baby is!!!
GLIDER: You will want a glider or some sort of comfy chair for Baby's room. It is sooo helpful for the late night feedings! I didn't spend a lot on mine and I love it. It is the Storkcraft Glider and Ottoman (Walmart.com).
PUMP: If you are planning on breastfeeding I would suggest this. Eventhough you are going to be a SAHM you'll want to at least get a manual pump. I am a SAHM and I registered for an expensive electric one. I'm glad I didn't get it because since I am with my baby so much I don't need to pump very often. If nothing else, you will want to have it on hand for when your milk comes in because you may become engorged. I also feed my baby a bottle every few days so he is accostomed to it for when we leave him with grandma.
There are also some things I WOULDN'T register for:
WIPE WARMER: I registered for this and didn't get it... and am glad I didn't. My baby doesn't care if his wipes are room temperature. I say unless you find that your baby is fussy at changing time, you don't need it.
BLANKETS: Unless its something you want for your nursery decor, don't register for blankets because you will get enough from your showers!
CLOTHES: You will get sooooo many clothes. You don't need to register for them!
BOTTLE WARMER: Unless you are going to be bottle feeding your baby, I would say you don't need it. I have found it's faster to just put a bottle in a mug of hot water than to wait for a bottle warmer to heat up.
BOPPY: A regular pillow works just as well. If you breatfeed you don't want to rely on a pillow to support a newborn anyway because it can cause you to slouch. It's better to hold your baby to the breast instead.
I know I wrote ALOT but I hope it helps you out!