Greeting Martin,
Skirball posted some good advice.
The Canon EF 24-105mm f/4L is a very high quality lens which runs about $1150 retail... BUT... this lens ALSO happens to be the "kit" lens included with a few of the high-end bodies including when the photographer wants the body but not the lens.
For example... when I bought my 5D III, I was not able to find anyone who had it available in the "body only" Canon packaging. I could only find the "kit" version which included the 24-105mm lens. I didn't want the lens as I already own quite a number of Canon lenses (I also own a 5D II).
It turns out most stores were advertising a "body only" price and when I called to purchase, what they REALLY do is remove the lens from the body/lens "kit" and send me everything else. They THEN sell the lens alone as a "white box" lens (the lens is not in Canon retail packaging.)
It turns out if you look at the cost of, say, a 5D III "body only" and the cost of a 5D III with the "kit" lens, the price tag goes up by about $550 to get it with the lens (even though the lens sells for about $1150 by itself.)
CONSEQUENTLY... there are many outfits that will sell you the "white box" version of the lens for about $700 (and sometimes less). That's a very good deal and it's not hard to find the lens at that price.
The other option that has already been pointed out is the 28-135mm ... which sells for a little less "new" ... just shy of $500.
On the 5D II body, a "normal" focal length is about 50mm. Anything less than 50mm is going in the "wide angle" direction... anything above 50mm is going in the "telephoto" direction. This means both the 24-105 and the 28-135 offer a wide-angle through moderate telephoto focal length. Neither are ultra-wide nor ultra-tele. They're both good general-purpose walking-around lenses for all-around photography.
The focal ratio on the lens name is the maximum focal ratio available. This is an indicator of how much light the lens collects. Lower focal ratios mean the lens can collect more light and it can create a shallower "depth of field" (when the lens is focused to any given distance... typically subjects a little closer and also a little farther will also be in focus. The "depth of field" is an indicator of the range of distances at which subjects will appear to be in acceptable focus. It's easy to create a broad depth of field... a bit tricker to make it narrower. Photographers (especially portraiture) like narrow depth of field because it means you can have a tack-sharp subject and a deliberately de-focused background which really helps draw attention to your tack-sharp subject.
The advantage of the 24-105mm is that it can provide the f/4 aperture at all focal lengths in the zoom range. The 28-135 is a "variable focal length" lens meaning it's wide-open setting varies depending on the focal length. At it's widest-angle setting (28mm) it's able to open up to f/3.5. But as you zoom in it quickly falls drops to f/5.6 (usually once you are just half-way through the range you already arrive at the f/5.6 setting... but that varies by lens.) Also the 24-105mm is an "L" series lens (the "L" designation is assigned to Canon's best line of lenses.) There are MANY subtle differences that you might not notice. Usually L series lenses tend to have more aperture blades and the blades are more well-rounded so the character of out of focus blur tends to look better. They're often (but not always) weather-sealed. The build quality is generally better. They certainly fuss over the optics a bit more to ensure better optical traits (distortion, vignetting, chromatic abberation, etc. tend to be controlled better.)
Even if the more expensive 24-105 is beyond your budget, you'll still want to be aware of the differences between lenses or you can get frustrated. I've observed people being frustrated with a "camera" because they cant get the look that someone else with a similar camera can get... and they don't realize that the lens was making the difference.
If you are passionate about photography... you may slowly want to accumulate higher-end lenses as your budget permits.
Tim Campbell
5D III, 5D IV, 60Da
