I've spent a lot of time in various places in europe, I usually have 2 cameras and a multitude of primes with me
Rome I shot on medium format for the most part
In barcelona most recently my kit was DA14, M28 3.5, M50 1.7, M100 2.8, m200 4.0 and 18-55 wr in case it was particularly brutal rain. I had My K7, K10 and a KM film body. I brought the K10 and left it in the Apartment the whole trip it was only there as a back up as barcelona is notorious for pickpockets and bag thieves. To be honest it's not an issue. as long as you are aware of your surroundings i wouldn't worry too much about theft (don't use a big bag that screams expensive camera for instance - I retrofitted a cheap canvas messenger bag with inserts from my crumpler that i transport the items there in. it held all the lenses and 1 body or 2 bodies and leave out the zoom and the 200)
what you will need most is the wide end and a low light lens. longer lenses are limited use in the narrow streets of Europe. I don't know what lenses you have but if i was trying to limit a kit i would go with a wide zoom (like the Tamron mentioned) and a fast 50 for interiors in churches/galleries/museums. I wouldn't bother with the 18-135 myself
Personally i'm a prime guy and all those primes weigh about the same as my Sigma 24-70 2.8 and grip needed to counterbalance it, but give me better performance and longer reach when needed
70% of the time though i shoot the 14 and the 28.
from the 100 trying to isolate from the throngs of people