Well Nikon (the "Nikon 18-200mm f/3.5-5.6G DX VR AF":http://www.popphoto.com/cameralenses/2763/lens-test-nikon-18-200mm-f35-56g-dx-vr-af-s.html makes are nice superzoom that is a good reference standard for superzoom. Iisn't fast and does have optical distortion (0.73% barrel at 18mm so you'll want to use DxO, vignetting everywhere below f/6.3 and it is good for 8x10 from 18-100, pretty good for 5x7 at 200mm).
As a general comparison of Popular Photography's subjective quality factor "SQF":http://www.popphoto.com/cameralenses/699/subjective-quality-factor.html, the really good "Canon 10-22mm F3.5-4.5 USM EF-S":http://www.popphoto.com/cameralenses/2235/lens-test-canon-10-22mm-f35-45-usm-ef-s-specs-page2.html had A+ performance through the 8x10 range as well and A performance through 11x14. I've printed it at 13x19 and have to say that is amazingly good). And the "Canon 24-105MM F4/L IS USM AF":http://www.popphoto.com/cameralenses/2235/lens-test-canon-10-22mm-f35-45-usm-ef-s-specs-page2.html which actually didn't do very well. The main issues being vignetting below F/13 at 24mm and F5.6 above that if you are using a full frame camera, but not with an APS-sized camera, so you definitely need DxO to correct that. Also it is slow at F/4, but does have stabilization. For subjectives, excellent up to 11x14 for all levels of zoom. I've seen that it is quite good in practice even at 13x19. As an aside, the "Canon 24-70mm F2.8L USM AF":http://www.popphoto.com/cameralenses/663/canon-24-70mm-f28l-usm-af.html doesn't have image stabilization, but SQF is way higher. It is excellent up to 11x14 at 28mm for a whopping 16x20 from 50mm and above).
But very versatile, if you have Canon, your only option (other than the 24-105 F4/L which isn't telephoto enough imho for true tourist work nor wide enough for landscapes) is to go to a third party lense. Here are some choices. As usual, quality can really vary here. The idea would be an image stabilized 18-200mm lense that is also image stabilized and supports the smaller cameras (the so called 1.6x lense factor or APS size sensor in the Digital Rebel XTi):
"Tamron 18-250 F/3.5-6.3 Di II Macro":http://www.popphoto.com/cameralenses/4110/lens-test-tamron-18-250mm-f35-63-di-ii-macro.html
This is a 14x zoom in effect and is in equivalence like a 28-388 on a regular 35mm camera. It is $500 street and light at 1 pound. The tested it on the XTi. The photo quality was really amazing. The subjective quality was excellent as long as you stay in the 11x14" reprint range at 100mm or below and and for 8x10 at 250mm. There is just definitely distortion (so when you use it at the extreme ends, you'll want DxO to fix this when actually printing). There is visible barrel distortion at the very wide 18mm (0.72%) and pincushioning elsewhere. There is also vignetting (or light falloff) around the corners if you are below F5.6 at 18MM or F/8 at 250mm so this is a lense that is only good at bright light or fixing it with "DxO":http://Dxo.com otherwise.
From measurements, the image quality was actually better than the Nikon and it is a little lighter, so not a bad choice except for the vignetting issues around the corners where you'll see light fall off (but DxO can fix that post photo).
"Tamron 28-300 F3.5-6.3 XR DI VC LD Apehrcial (IF) MACRO":http://www.dphotojournal.com/tamron-28-300mm-f35-63-xr-di-vc-ld-aspherical-if-macro-lens/
How is that for way too many acronyms. It was just announced and is probably not wide angle enough, but it does have vibration compensation on it. It is more designed for full format (that is like the Canon 5D) cameras, so will be big.
"Sigma 18-200 F3.5-6.3 DC OS":http://www.sigmaphoto.com/news/news.asp?nID=3277. This is announced but doesn't seem to be available yet. The big new thing is optical stabilization. It seems the direct competitor to the "Nikon":http://photo.blogger.ph/2006/10/09/sigma-18-200mm-f35-63-dc-os/. Various sites like "Mobileplanet":http://www.mobileplanet.com/p.aspx?i=141773 claim it will be available June 16th or.
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