I don't think there is the ONE focal length / lens that makes everyone happy. That's why it is so hard to answer the "what lens should I take" question.
It very much depends what the photographer wants to do. Some of the best photos I've seen have been shot with a simple 35mm or 50mm lens (= 24mm or 35mm on cropped sensor).
- Landscapes and Interiors (churches, museums,...): Wide angle down to 10 or 12mm (on a cropped sensor such as the 400d).
- Bazaars & Markets: How closely do you approach people / what's your 'comfort zone'? Some people only shoot from a longer distance (telezoom), others like to be "right in the middle" – in which case you'd want a good wide-angle.
- Full face portraits: medium focal length with a large aperture (f2.8) – i.e. a 28-70 or 28-80. Environmental portraits, the max aperture becomes less important.
- Wildlife: "the more the merrier" but at least 200 or 300mm on a cropped sensor I'd say. And if someone is into bird photography, focusing speed is important as well.
The second hand market has some oldies but goldies – such as the Tokina f2.6-2.8/28-70. I think it came out about 10-12 years ago, not available new anymore. It's missing the "made for digital" tag, hence should be available for a bargain – but I'm sure its optical quality is comparable to modern lenses.
Try to find a good photography shop with a nice selection of lenses, and ask them if you can leave your passport / driving license / bike parked in front of the shop & take one or two lenses for a little walk around the block to check them out (focal lengths, focusing speed, focus accuracy,...). Take test shots with YOUR camera at different focal lengths and then have a look at them on a computer. Try to detach yourself from "wow" factors & ask yourself if you'd actually USE a certain focal length on your travels. (I've got a 12-24 on my full frame body – other travellers often envy me for this amount of "wide angle", but I actually find it's very hard to use properly. If I had to do it again, I'd probably leave the 12-24mm at home and take one or two fast prime lenses instead – I just LOVE shallow depth of field / selective focus).
Unfortunately, approaching a photo store for this kind of "favour" is often difficult these days – too many people "test in the shop & buy on the web" and of course shops are aware of that. That's why I try to buy most of my gear in my local shop & build up a relationship with them.
(That said – I've heard from one traveller that he ordered 5 lenses on Amazon (Germany), tried them all out & sent 4 of them back – no problems).