Well personally I love the Canon 17-85 IS, the Sigma 18-125 and the Sigma 18-200 on 1.6x bodies for this sort of thing. This summer I took my 20D with the 17-85, the 70-300 IS, and the 15mm fish to Italy for two weeks. I used the 17-85 about 90% of the time, the fish maybe 8% and the 70-300 about 2%. Especially with the IS on the 17-85 and the 20D's superb high ISO performance, it's all you need in even astonishingly low light.
The two Sigmas are terrific travel lenses in their own right, and their lack of IS was the only reason I traded for the 17-85. If and when the new 18-200 OS version ever starts shipping, I will try it as an alternative.
You can see some of the casual travel sort of stuff I did with the 18-125 on the 20D here
(all with EXIF).
I like small and light and versatile for travel. I even recently traded the 20D for a 400D for this purpose because it's usefully smaller and lighter (not to mention quieter). The last thing I want to do (well, maybe not the last
thing, but close) when I travel is to schlep a lot of heavy gear like my 1-series bodies and L lenses.