For outside shooting, a circular polarizer is nice. Hoya makes great ones if you get the multicoated filters. They are pricier, but reduce flare big time.
Some people get a uv filter (again, if gettting hoya, get at least the hmc multicoat) to mainly protect their lenses from scratches. However, any additional piece of glass you place between you and the picture *could* potentially effect image quality. However, the degradation would be extremely tiny if you buy a nice multicoated filter. I think if you get the 6 coat hoya it transmits 99+% of the light. If you get the cheap uncoated it only transmits like 80 something percent of the light. So don't cheap out if you choose to do this.
While looking at polarizers look at the moose polarizer by hoya. Lots of people shooting landscapes use them and they 'warm' the picture somewhat.
http://www.photosig.com/go/photos/browse?id=28636
When you get additional lenses, you may try a hoya r72 infrared filter if you are so inclined. It has been reported that the kit lens does get a hotspot however, so you'll have to figure out which nikon lens does not get hotspots while shooting infrared. I have no experience with nikons and infrareds, just reported what I've read.
Finally, for better macros you should look at the raynox 250. Lots of people will mention tons of other ways to do macros, however probably none are cheaper or easier. Read the reviews at amazon.
http://www.amazon.com/Raynox-M...iameters/dp/B0002YBXBY