I Live in Europe, and have a 6D, and like you I like my primes, and for a holiday without doubt I'd leave them all at home. You'll be seeing lots of landscapes, interiors, and small narrow congested streets, so take a wide angle zoom, 16-35 or a 17-40, 9 times out of ten it'll be on your camera. If you have to have a prime, take a 40mm pancake, on a 6D it's a killer combo for street, and something with a bit more length for picking things out, possibly a 70-200. Personally, for my 'days out in Europe', mostly with the family, I take the 24-105L, a much maligned lens, and for the life of me, I don't know why, it covers the most useful range, and can be sharp as a tack.
As for security, it's as secure as anywhere in the world! The only common sense thing I've ever done is remove the 'CANON 6D' neck strap and substitute it for something more subtle ( and a LOT more comfortable! ), one advantage it that it stops you advertising that you're carrying something worth nicking, but rather it saves your holiday being ruined by camera nerds wanting to talk to you about cameras
Personally the second I see anyone with the OE CANON neck strap, no matter what model it's advertising or how much it costs, I know they don't spend much time using it....
6D + 24L 50L, and 100L -Free (owned)



