Kit lens replacement for trip?

In a few weeks I'll be heading to Lake Tahoe. The kit lens on my rebel xt has been plenty useful for the year or so I've had it, but I'm wondering if I should grab a better lens before the trip. Is there something that has decent wide and zoom, for walking around, and landscapes? Or is there no such thing?

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I'd get teh tamron 17-50 before that sigma, but yes there aren't many do all lenses.

the it lens + 24-105 f/4 IS or kit + 70-200 would be good combos, but if you like the wider side you might look at the 17-55 f/2.8 IS, a bit pricey but great lens. te downfall is obviously the limited range.

Ive been looking at the Tamaron 18-250 for an all around lens, good price at around $500 and f3.5 from the remarks of user`s seems not too bad.

The Tamron 17-50 is not a landscape lens - way too much CA - otherwise it's near perfect. If money is tight I would go with the Sigma you mentioned or the Tamron if you want very sharp pictures, however, if it is not then the Canon EF-S 17-55 2.8 IS is unmatched in this range. It delivers fantastic detail, very well controlled CA, is very sharp at 2.8 and the show stopper is IS. The EF 24-105 f/4 ISL is also a great lens with plenty more on the longer end - the only two drawbacks is that it is not very wide and not so great in lower light.