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 Denmark, Holland and Belgium only tourists wear them, but it is so easy to take a tumble and hit your head, particularly on a loaded bike.
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 in Amsterdam.