I think I have a telepathy with Duncan. Its only who gets to answer first.
By far battery is the first thing you want to add.
I hope you have got your extra batteries of ebay and not from the camera shop or the Original Nikon's. As otherwise you might have paid a lot more. I get a typical extra battery for my Canon XT or 30D for about $10 including shipping and handling and boy you need those extra ones on every special occasion. Just checked ebay, a quick find is here for 2 packs (Canon is a lot cheaper. You should have asked me before buying Nikon
):
LINK
Battery grip would be another good option. Because it combines two batteries, your usable time is extremely prolonged and you not only have better grip but better option to click the shutter in vertical frame format with the extra shutter release.
Extra memory is your next needy on the list.
You can substitute this with a 2.5 mobile hard drive memory reader that is a world by itself considering needed storage it provides you (On a journey for eg). I believe that was nirofo that gave a very good suggestion sometimes back in forum for less than $100 to get a mobile hard drive of say 120GB put in the external hard drive self powered case, if you are willing to put it up together yourself (You need to be a bit handy with computer componant but quite simple) So check the forum. Alternatively here is an example of what I suggest:
LINK
Next is the UV and Circular polarizer filters.
Next is a good and comfortable carrying bag for most gadgets needed on a typical shooting secession.
Next off camera flash or flash cable especially for your portrait needs. Alternative is flash slave. Depending on how much you want to put down a budget, you can go for very simple manual slave or top of the line TTL off camera wireless flash with slave.
Next is the other extra lens,
next L I S T G O E S O N & O N . . .