I usually travel with a carry-on bag and a small personal item (smaller than what would be allowed). I've done this to Europe in both summer and winter. The key to the Rick Steves approach, which I largely follow, is to do sink laundry every day, or nearly (perhaps not on the night before you pack up and leave for the next destination).

That means that the amount of clothing you need is the same for any trip of three days or longer unless there's a change of seasons, and maybe even then. My last summer trip to Europe included an excursion to high altitude and I still managed to get everything into an ebags Weekender and an Eagle Creek guide bag.

With one exception. With a sport jacket included, the Weekender was too thick for an airline sizer, so I carried the jacket over my arm on air-travel days. On train-travel days I could fit it into the Weekender.