What lens do I need for the Nikon D3100?

My girlfriend has a Nikon D3100. She is absolutely in love with it and I don't know enough about what is good. She has a couple of lenses and she doesn't like to change them out all the time. Which would be a good lens to do everything that she would need. I'm deployed and am trying to make it a nice surprise for her.

No lenx exist that offers both a long range and is high image quality and is cheap. You can choose to
Long range + Cheap = The Tamron 18-270mm. It is a pretty decent lens at 3.5-6.3 but you'll notice that it certainly doesn't offers the best image quality
High Image Quality + Cheap = Primes, Primes do not zoom. But they go very wide open and offer some great glass. Primes are lovely pieces of glass that might not be as versitale as a zoom but quality is quite something special
Long Range + High Image Quality = >$1000 Nikkor glass. Yup the stuff pro's use.
The problem is simply unless you know what your Girlfriend really WANTS it is hard to buy a fitting lens. What if you buy a 50mm prime and she rather wanted a 300mm zoom. What if you buy a 18-270mm but she rather wants a bit more image quality.
So really buying a lens for a person if you don't know what kind of photography pulls them the most is extremely hard.
how about this a flash, indeed a flash, no more need for the popup flash instead a nice big flash. Giving LOTS of light and allows for keeping the ISO low. Meaning better image quality with the lenses she already owns. So buy her a nice Flash, or how about a Tripod, a $150-$200 tripod for instance. Nice and Sturdy and fit for the heavier work.
And well she has quite a few lenses so how about a nice camera backpack. So she can take her lenses with her much easier and her other tools.
Or how about a Monopod? Again the gains of a tripod but smaller and easier to take with her.
Or… Tell her she can get 10 images printed out big professionally and framed. So she can have 10 pictures hanging from the wall.
I know I would be happy if I did get that as gift! There's something about printed out images.
So rather then taking a chance on buying something she probably doesn't use. How about buying her something that helps her with her photography. The tripod/monopod/flash or yes the big *** print out!

Expensive yes, but the 18-200 mm will cover most of what she'll ever use. I use the 18-200mm 95% of the time.

Just so that you and your girlfriend will know, the more versatile (more zoom) a lens has, the poorer it is optically. The first one has the longest zoom in the entire dSLR world.
http://www.bhphotovideo.com/...rtclk=sort
This one is cheap and if your girlfriend does not have it yet, she may appreciate it more for it's superb background blurring and low light shooting powers.
http://www.bhphotovideo.com/..._50mm.html
Perhaps after using that, she'll change her mind about swapping lenses.