I used to use RS Civita Day Pack before security requirements turned against backpacks in museums. Currently I use RS Veloce Guide Bag. Pros - It has been allowed in Europe museums. Cons - No exterior pouch for water bottle, also new iPhone XR too large for front pocket. (Actually XR fit tightly in front pocket which is now ripping). I'm considering switching to Travelon shoulder bag. I'd appreciate input on what is functional as a day bag for men and allowed into museums.
Museum rules vary. When I traveled around the eastern part of Germany in 2015, I was required to check my purse in most if not all of the museums I visited. It happened again last week in southern Spain (can't remember where).
For those who are not aware and also a reminder:
No bags are allowed into the Borghese Gallery.
Girasole is right. I carry a small cross-body bag when sightseeing, and had to check it at the Borghese.
If you have a few extra bucks to spend - check out the Red Oxx Gator Bag. I bought one for my husband for an upcoming trip. I can easily test out the outer pocket for a water bottle for you if you are interested. It's definitely a manly bag, comes in many colors. Makes a great underseat flight bag. Padded to protect electronics. Red Oxx just improved this bag.
Besides Travelon, there is Kipling - very lightweight, a lot of pockets, unisex bags and backpacks that offer dark color choices. Kipling bags are sold at TJMaxx, Macy's at decent prices.
Pacsafe is another, but the ones with the steel mesh in the panel and strap can make it heavy after walking around with it all day.
Hedgren is another very good company for unisex travel bags - backpacks and day bags. Also very lightweight, many pockets, good strap. Sometimes you can find certain styles at TJMaxx.
Consider the Pacsafe Vibe 325, it's low profile yet carry's a lot of stuff, great security features. I love mine and never travel without it.
My Patagonia Regugio backpack was not a problem in Scotland but that was there and that was then.
For everyday carry, I can recommend Tom Binh's Ristretto (huge, 9L, $150) or Café (small or medium). Great products, beautifully crafted, and there are several excellent reasons to buy stuff from TB. I have a PacSafe Intasafe 25121 small shoulder bag that I no longer need. Has water bottle pocket and many interesting features.
We've had a Metrosafe LS200 medium crossbody bag for 12 years that my husband has carried on our trips abroad (and we've loaned out to friends). Some folks object the weight of Pacsafe bags but that hasn't bothered him so as it's showing some wear, we'll probably replace it with the same or similar.
The only places he has ever had to check it have been the few museums, like the Borghese in Rome, where you have to check almost any type of bag at all.
I don't own one of these, but for a while their adds were popping up all over my Facebook feed.