You need waterproofs just
in case. For Europe we take a lightweight (two ply)
Goretex jacket, lightweight waterproof over-pants,
waterproof shoe covers, and waterproof gloves. If you
are camping, you would need a heavier jacket.
We wear helmets. In most European countries only tourists
and sports cyclists wear them, but it is so easy to
take a tumble and hit your head, particularly on a loaded
Apart from that, it's the layer thing. I usually wear
Polartec 100 pants and top. I find it adjusts to a
range of temperatures and is quick to dry. If it is
warm enough I'll wear shorts and a t-shirt.
We take smart-casual pants and a couple of tops for
going out to restaurants, and a spare pair of shoes.
Smart casual wear is acceptable everywhere these days.
We also leave a bag of clothes at the start/end accommodation
with the cycle bags. That's useful if that is a major
city and you plan to spend a few days there as we did