When you travel with a bicycle as your primary mode of transportation on a European vacation it's all about small, lightweight, and practical things.
Noise-cancelling earbuds are a must. I use Beats Studio Buds or Jabra Elite Active 75T earbuds (tho the latter are sadly no longer made). Both are great for the flights and for basic use.
Compression stuff sacks are also essential: pack more things into the bags that live on the bike. Sea-to-Summit are my faves, REI's house brand are a close second and far more affordable.
I have a Sea-to-Summit Ultra-Sil day pack that lives in a bike bag for times when I need a backpack. I prefer to ride without anything on my back, but this pack is handy for when I hike or go exploring off the bike.
I bring Chico Bag shopping bags with me, as well. They work well for grocery runs.
Microfiber towels pack small and are great for many uses. I often trim them to smaller sizes for different uses.
Also a huge fan of Voile Straps to help affix anything. I use them to strap my Teva or Chaco sandals to my bike rack, to affix my bike onto train racks, and to help keep my bike less messy in its travel case. I've also used them to temporarily mend bags that have gone awry.
I carry a small amount of Gorilla Tape with me, as well as Gear Repair fabric mending tape in various kinds and sizes (e.g. Ultra-Sil, stretchy Lycra, Gore-Tex shell, et al), as well as some small scissors. I've also used NoSo patches for the same mending purpose.
I tend to buy sunscreen (and Euro sunscreen is a lot better and more advanced than any U.S. formula), toothpaste, and soap in Europe rather than bring them from home.