400 dollars or pounds? That makes a big difference.
If dollars, then there isn't much you can get, perhaps a cheap Tamron lens like the
Tamron 18-200mm f/3.5-6.3 Di II lens. However, this is very cheap, so quality will suffer quite a bit. Also, this is a rather "slow" lens (meaning that it will take a longer shutter speed to create a "proper" exposure than a "fast" lens like an f/2.8), even to the point that, at the far end of the lens, your camera might not be able to autofocus. Try it out in a shop before you even think about buying it. Take test shots and look at them on a screen, print some cropped photos out, etc, to test the lens' sharpness and colour saturation. Also, unless the lens has an in-body focus motor, autofocusing will not happen at all.
If it's £ you're talking about, then your options open up a bit, but not much. The
Nikon AF-S 18-200 f/3.5-5.6 VR lens is quite possibly the best super-zoom lens out there, but it carries a hefty price tag, £428 to be precise. Quality is actually fairly good, and the f/5.6 maximum aperture at 200mm and the AF-S model means your camera will definitely autofocus with it. Vibration Reduction also helps when shooting wide-angle in low light, or telephoto in dwindling light.
However, all of these lenses are not as good as buying two different lenses that add up to the same price. There will be more distortion, worse results at the extremes of the lens (very wide-angle and long telephoto), and are quite slow lenses.
But as a "quick fix" to the swapping lenses problem, those two are good. Just make sure you are happy with the results they produce before buying anything. Enjoy!