So much fun! I love Europe!!!!
I haven't been on a Rick Steve's tour, but I've been all over Europe many times, and keep it to one bag each time we go. My suggestions of what we normally bring :
-comfortable walking shoes (tennis shoes or other)
..wear one on the plane, and bring a second pair that you can wear to a nice dinner, etc... I find that dark shoes for both go a long way for dressing with versatility. I have black sportier tennis shoes that can double as both, for example, in case one pair is not agreeing with my feet one day. You'll do a LOT of walking.
-Bring pants that will last a couple of days/wears without having to wash them (or will hand wash well in a sink). They only people that wear shorts in Europe are Americans (for the most part), so it depends on how interested you are in "blending in". I would suggest more pants, because if it gets unexpectedly cold, you're not stuck.
-Bring a wrap/shawl of some sort for chillier evenings. It can double as a skirt in a pinch.
-I find that sweaters wear better than cotton for multiple days. But for shirts, plan the same approach as your pants. Things that you can wear many days, wash easily in a sink if need be, and will go with all of your pants so you can rotate outfits.
-socks, I bring 3-4 pairs and wash them in the sink after I wear them each day.
-underwear, same thing as above.
What fun you'll have! Hopefully that's helpful!!!