Given that it's been helpful for me to read what works and what doesn't on this forum, I thought I'd share our recent trip packing strategy.
There were four adults on this trip: M63, F55, F24, M20. Given that this trip was a big splurge for all of us and us ‘rents were covering a lot of the costs, our plane tickets were super economy (no checked bag allowance). The majority of our trip was inn-to-inn supported hiking (our luggage was moved for each day) with a few days in Rome to start.
We each had a personal item (that most likely exceeded United's dimensions for a personal item, but no one even blinked) and a carryon duffle/backpack. The 'kids' each had Deuter Aviant Pro-40 duffles, I had a bargain basement Hyeagle 38L travel backpack, and spouse had a somewhat better but still bargain basement 40 L travel backpack. My personal item was an Osprey Daylight plus and then I also had a Fjallraven High Coast Pocket which I used “around town”. I packed: 1 pair flip-flops, 1 pair trail shoes, 1 jumper (Title 9), 3 tanks (2 cotton, 1 tech hiking), 2 merino t-shirts (Bombas), 1 pair shorts, 1 hiking pants, rain poncho, 6 underwear, 6 socks, 2 bras, sleep shorts, sunshirt, lightweight hoodie, sun hat, swimsuit, power adaptor, clothesline, toiletries (Dr. Bronner's liquid soap, etc), pack towel. Also included hiking notes, maps, copies of travel insurance/passports, and printed out tickets to colosseum and Borghese gallery. My daypack had a pashmina, travel pillow, kindle, phone, water bottle, charging cords (in a BlueQ pouch), makeup (another BlueQ pouch), etc. I have a “bra purse” where I kept my passport and cards — I pinned this with a diaper pin to the inside of my Fjallraven rather than keeping it under my clothes — even if someone had opened the Fjallraven they could not have just pulled out the pouch with the passport/cards. On the plane I wore lightweight hiking pants, compression socks, Adidas town shoes, a tech polo, bralet.
My bag kinda sucked because it had no structure at all, but packing cubes definitely helped. It also had external compression straps, which helped maintain a bit of structure when I was wearing it. And it was fine for what it was — I think the longest carry was through FCO from landing through immigration to the train and it wasn’t great but it wasn’t horrid. Would it have been better with a waist belt? Absolutely. But then again it was nice not to have to deal with one -- tradeoffs! It did have a sternum strap which I think really made a big difference. And even with that I really liked having a backpack rather than wheeled luggage — I just felt so much more mobile. I ended up not using the rain poncho (we only had light sprinkles one night) and the hoodie (though if it had been a few degrees cooler or a bit more windy then it would have been used). I do wish I had packed Chaco's instead of flip flops only because we did do some rock-jumping-into-the-Med and the rock clambering would have been much nicer in those than bare feet (which limited how much clambering we could do).
One huge benefit was that our plane back to the US was delayed and our nice connection cushion shrank to ~75 minutes. Gulp. But we were so mobile that the power hike through immigration and then back through TSA and then to our gate at EWR was stressful but also manageable....