I’m hesitant to advise you what you should bring, but I’m willing to tell you what I brought and how well that worked. I went on the 14 day Spain tour in early September, so the weather may be different for your tour.
For clothes, I brought 2 pairs of polyester travel pants, 3 polyester polo (golf) shirts, 1 long sleeve polyester travel shirt, two pairs of wool hiking socks and 3 pair of polyester underwear. I could have done with less. I also brought a light polyester fleece pullover and (thankfully) a rain jacket. I brought, but never used, a stocking cap, a neck gaiter, and a pair of fleece gloves in case it got cold.
I also brought a universal sink stopper and did laundry in my bathroom every night using the hotel’s shampoo or body wash. Wash, soak, rinse, wring, wrap in a towel to remove remaining water and hang on hotel hangers to dry overnight. My quick drying polyester clothes were always dry in the morning. Most days, I just put on the freshly laundered clothes and left the packed clothes in my suitcase.
For me, choice of materials is key. Quick drying polyester travel, backpacking or athletic (golf) clothes are good. Wool is also good because it doesn’t collect the bacteria that smells bad. That means wool garments can be worn repeatedly without being offensive. Wool takes a while to dry, so if I feel a need to wash it, I try to do it at a location where I’ll be staying for a couple of nights. My preference is for casual looking styles that look okay on walking tours but still look acceptable to wear to dinner. Being a guy helps in this regard.
My shoe of choice is a pair of Keen sandals. I’m from Seattle, so I wear sandals with socks (even in the winter). It’s a Seattle thing: you might not understand. One pair of shoes was enough for me. Flip-flops don’t work with my socks and I can’t wear them on sight-seeing walks.
I never bring t-shirts with me on vacation. Should I feel a need, souvenir t-shirts are always available at the gift shop somewhere on the tour.
My preference is for a soft-side carry-on roller bag. I find it easier to carry my roller-bag than to drag a back-pack. I bring an easily packable day-pack for walking tours and general daily use as a man-purse. No pads, no stiffeners, lots of compartments, folds into its own pocket. I’d never use this pack for hiking in the mountains, but it works fine for sightseeing. I’m heavily dependent on my smart phone for GPS maps and directions (especially back to the hotel), for Google Translate, and for a note-taking app that came with my phone. A dead phone battery is grim. My day-pack always includes a USB recharger battery and cord.
Every trip I take is a learning experience. I try not to regret forgetting something I need or bringing something I don’t. I try to learn from my mistakes, but my vacation is too important to dwell on them. Don’t be afraid of making a mistake: just try not to repeat it. Whatever you bring (including your luggage), you’ll do better next time.