I do agree with jwenting on the view that you should bring two lens instead of one. On the notion that different lens serve different purposes and the also the extreme odds of one the lens being damaged during the trip.
If you are comfortable with Third Party lens, ie Sigma, i can offer you these two suggestions, which I am using for my overseas trip as well.
1) Day time walk about
Sigma 17-70mm f/2.8-4 DC Macro OS HSM.
Pro: Wide Angle at the 17mm ath F2.8, for tourist in Europe, my guess is that you will be taking lots of scenery and building pictures, the wide angle function will be useful to you. At 70mm, you will be at F4, which is still not too bad. Light weight and relatively bang for buck.
Con: limited range at 70mm. Unless you are very sure that you will be taking wild life animals which you will need the extra zoom, i will advice you getting a shorter BUT faster lens instead.
2) Night time lens
Sigma 30mm F1.4mm F1.4
Pro: Fast lens at F1.4, excellent lens for low light / night time, at 30mm, it may not be ultra wide as the 17mm, but you still can get a decent amount of detail. Small & light weight
A good secondary standby-lens to the normal walk about lens.