Lens for trip, filters

I'm about to take a 6 month world tour, I'm going to be in cities, countrysides, jungles and deserts. If you could bring ONE lens with you for a trip like that, what would it be? I have a Rebel XTi.

Also, in case I can't afford said lens (which I think is a big possibility), I may stick with the kit lens. I am really new to photography and I have noticed that in a lot of my pictures the colors of buildings, people, rocks, etc. are great, but the sky is washed out and light blue. Then if I modify it on my computer the sky looks good and the rest of the image looks too bright. What to do? I had someone tell me once that I needed a filter. No clue what that means... sorry. Could someone tell me what exactly I should get to go with the kit lens? (type of filter, size, etc.)

Thanks!

One lens on a world trip is problematic and you'll get lots of suggestions, I'm sure. I prefer to bias toward the wide end, so for the Rebel I would have to have something in the 16-17mm starting point, maybe wider.
However, as far as filters go--a circular polarizer is what you need. It will darken skys, eliminate reflections and saturate colors, if used properly. If you have time, read up on how to use it before you go. And, buy the best one you can afford. B+H or Heliopan are the best. If you end up with a very wide lens, consider a slim filter. I understand the Heliopans are threaded so you'll be able to attach the lens cap. B+H are not so you have to use the push-on cap that comes with them--not as good. Size will depend on what lens you end up with as they vary greatly. Thread size is generally indicated on the front of the lens with a symbol that looks like an O with the slash through it followed by the filter size, 58mm or 72mm, etc. I don't have the kit lens so can't tell you what it is.

I'd suggest you get a Circular Polarizing filter and a lens hood for your kit lens and go with those. The filter will help deepen the blue of the sky in a lot of situations.

A graduated neutral density filter is even better at this, but requires a holder and is a bit more involved to use than a Circ Pol. Cokin is an inexpensive brand to exepriment with. They make two densities of graduated ND filters (I think they call them 'Graduated Gray' or something like that).

In post processing in your computer you can also just select the sky area in your image and adjust only that with most image softwares.

Another way to do this is to double process your images in your computer. Make a copy of the image and adjust it until the sky looks right. Make a second copy and adjust it until everything else looks right. Then overlay one image on the other and erase the 'incorrectly adjusted' area on it so that the other image will show through.

You might want a second lens for certain situations on a trip like that, but you haven't given us much info on what you want or like to shoot, so it's not possible to give very helpful advice.

Personally I would never travel with just one lens. That would defeat the whole purpose of an interchangeable lens SLR camera. Might as well just take a smaller point n shoot, if unwilling to carry and use a second or even third lens.