B/B,
Just from perusing, and not a rigorous rec'd, I found a few zoom teles you/your wife might want to consider. None are the L Glass family,
as I would think you would want Image Stabilization for longer focal length shots, and the L Glass is pricey in some formats.
Not in any order, but they all should work on your Rebel XSi:
-18-200mm
EOS (SLR) Camera Systems - Standard Zoom - Standard Zoom - EF-S 18-200mm f/3.5-5.6 IS - Canon USA Consumer Products
-70-300
Amazon.com: Canon EF 70-300mm f/4-5.6 IS USM Telephoto Zoom Lens + Deluxe Accessory Kit: Electronics

18-135mm:
Amazon.com: Canon EF-S 18-135mm f/3.5-5.6 IS UD Standard Zoom Lens for Canon Digital SLR Cameras: Camera & Photo

No Canon xpert, as I barely have my new 40D out of the box, and no Canon
lens maven. My point is a tele much past 135 requires a mono or tripod, or
fast shutter speed and sniper like holding control. And, unless your wife is
into swapping lenses at the blink of an eye, why not go with a compromise
med wide through med tele lens that will "cover" about 90-95% of the shots
you want to capture? And, remember the digital lens multiplier vis a vis the
"capture" vs film cams and lenses, so those longer lengths on even a 135 zoom,
are a pretty long reach.

And, I will quote my initial comment post in this thread:
"Pretty good pics; maybe a future telephoto in your cam bag?"

Cool trip...
GL,mD