A good thermometer
I went with one of those infant first-aid kits instead, but you know what? The thermometer that comes in those is pretty darn crappy and I hardly ever used most of the other items. A quality thermometer would have been a much better option, especially since it's something you'll probably use over and over again throughout the years.
When I had my first child I thought those nasal aspirators that you suck air out of to clear your baby's nose of boogers were so gross, but the more traditional bulb syringes are actually really difficult to use in tiny noses and are prone to getting moldy.
I thought these were so frivolous until I had a winter baby and found myself desparately trying to warm up cold wipes in my hands before every diaper change. Now I wish I would have just gotten the wipes warmer.
Manual breast pump
Oh, how I wish I would have had a manual breast pump. I had the big, heavy electric pump, but I only had to pump really often for short periods of time with either child. If I had had a manual pump though, I could have used it to empty a breast when my baby didn't, which would have helped keep my supply up. Plus, they are more convenient for traveling and road trips.
I never purchased stroller accessories, but there have been many times I would have been happy to have a cup holder, snack tray, bunting, extra hanging toys, etc. It would be pricey to buy them all at once, but if I had registered for them, I'm sure several people would have bought one or two pieces here and there.
No, not bottles and nipples and stuff — most people register for that stuff. I mean supplies for when baby starts solids. Sippy cups, those cute trough or pocket bibs, spoons, sporks, forks, plates, etc. And lots of them too, because babies, and then toddlers, eat often and nobody has time to wash dishes six times a day.
A stretchy carrier
I absolutely love baby carriers and still use one with my toddler, but the only one I ever registered for was a traditional front carrier with buckles. I wish I had also registered for what's known as a stretchy wrap. It's a long, stretchy piece of fabric that you snuggly tie your baby to your body in, and they are so awesome for newborns and young infants. I bought a used one for my second baby and used it so, so much!
Clothes in various sizes
I didn't register for much clothes for my babies. Everyone said that people always buy clothes for babies because it's so cute, but I didn't receive many clothes for either of my kids. In retrospect I should have registered for at least a couple of outfits and some basics in various sizes. Baby clothing is expensive!