I think about what I'm going to be doing when packing and plan accordingly. I'm a huge fan of lists so I first list out my essential toiletry items (or items to buy once you arrive since plane rules are strict nowadays on quantities), then my essential basics like socks, underwear, etc., then I estimate how many casual and dressy clothes I'll need and pack a good ratio of those and include at least one hot weather/cold weather in each category. Finally, I pack smart tennis shoes, dress shoes (preferably ones that work with anything) and a bathing suit. I would pack a light jacket or heavy sweatshirt in case of cold temperatures too. Hope this helps. I still end up overpacking lol but usually this works out okay. I couldn't imagine taking one suitcase to Europe! Then again, I have kids so I pack for the unexpected lol.
Hope you have a great vacation!