best daypack ?

I can't advise on a bag, but I would suggest making sure it is light weight. I also suggest trying to get a thin wire to wrap around and through the straps. I have heard and read that some scoundrels will use a blade to cut the straps to grab your bag. If you reinforce with a wire, it won't be easily cut through. Also, I recommend a backpack over a messenger bag because you can better distribute the weight if your bag should become heavy, plus using the two straps will make it harder for the theif to grab you bag versus if you only had it on one shoulder.

If pickpocket safety is your main concern I'd go with a messenger bag. Easier to keep your hand and eyes on.

I don't have any specific models in mind, but do have a few criteria.
Lightweight when empty for obvious reasons.
Zip top (perhaps with a buckle/snap flap over the zipper but not necessarily). Too many bags just have the flap, leaving the contents vulnerable to rain or theft.
Not too large. You want to be able to carry it easily into sights with size limits on bags
Has smaller inside zipper pockets (I sometimes safety pin these closed for passports or credit cards).
Comfortable when worn cross body (having it hanging off your shoulder makes it easier to grab so crossbody is the way to go)

My preference is Patagonia bags. How many travelers have had their packs stolen by slashing straps? I guess you could go the extra route...seems profoundly absurd to me. Manage pickpockets though.

I opted for black pacsafe sling bag that will sit.in front on my chest....very light and roomy for what I need...all bags attract atte tion regardless of brand...I was sitting on the fence but then I found it online for 50% off which sealed the deal.

9x10.5x3 Lily Bloom wide strap across chest and a flap the length of the bag (9). At home the large zipped pocket would be the back of the bag. I place the zip and snap on the flap against my chest. My wallet with limited valuables is deep in the bag under the flap. The large zip pocket is used for non valuables. On the Tram in Lisbon I was in and out of my non valuable section. We were being scoped for a crime by a clean cut well dressed thief. His successful target had a simple zipped bag against her back side and carry two coffees. There is no way he would have had access if the bag was against the front of her body with an arm on it.
I am changing to a Tignanello 12x12x3 with several zipped areas including a deep zipped area. The large flap on the Lily Bloom is a crime saver but it drove me nuts.
My husband wears a addidas grab bag or swim bag with a zipper. He wears the zipper against his body to deter thieves. He does not carry valuables in this bag. No attempt has been made to cut the simple shoe strings. Limited valuables are in zipped front pockets.
We lock excess valuable in the room safe. If necessary we both wear valuable pouches around the waist.

The Osprey bags look like they have sturdy straps as well as a waist belt, so I would not be too concerned about straps being cut. And if you're not carrying valuables in it, then there's no real worry. The most vulnerable time is when you take it off and set it down for any reason (including to get things out, eating lunch, onboard a train, etc). When I wear my cross-body bag, I never take it off.

But I would secure cash, credit cards, passports, and expensive smart phones closer to the body and preferably under a layer of clothes, in a combination of money belts, neck wallets, and maybe one cross-body bag. It also sounds like you've been making one person carry the gear for two, but in the case of cash and ID, you should each have some. See more tips at https://www.ricksteves.com/travel-tips/theft-scams.

I would recommend United By Blue bags. They're cheaper than Patagonia and Osprey, super light and perfect storage for that kind of daypack use (patagonia and osprey would be better for a longer hike). And definitely a backpack! You can get a small lock/wear the backpack in front in larger crowds, in high pickpocketing area. You can order them online or get them at MEC.

Wow ! Thanks for all those answers (and the others ones that will come !)

All those tips make me more nervous about our trip to Barcelona ! We will be 2 adults and will have our 2 kids (5 and 9) with us. That mean that maybe we wont be 100% focused on our bags.

We have a money belt and a small bag that have a zipper and a flap. Maybe will keep our old Osprey Talon....is we could found a way to make it more secure it sould be great. It doesnt have zipper but more shoestring like....

Leaving everything at our lodging may not be the beat. We wont have an hotel room, we will have an appartment. There's no safe in it it. Should we still leave our Ipad and passport there ?

My husband and i LOVE "Sea to Summit" products. We each use one of these as our daypacks (they weight NOTHING):

http://www.seatosummitusa.com/product/?item=Ultra-Sil%26reg%3B+Day+Pack&o1=0&o2=0&o3=377-36

They are water resistant and have served us very well on a few European vacations. They now make a "messenger"-style bag from the same material.

We also use the Sea to Summit pack towel -- very small and absorbant.

They are not slash-proof or anything, but we don't put anything of value in them except, perhaps, our compact Zeiss binoculars and name-brand sunglasses -- both of which, if we lost, would not ruin our trip. I carry a very small, black, Travelon cross-body purse (similar to PacSafe) for my real valuables & iPhone.

https://www.amazon.com/Travelon-Anti-Theft-Convertible-Crossbody-Waistpack/dp/B00SU4XU64?th=1

I never remove my purse when out and about (transitions to a fanny pack when I sit at cafes, etc), and my husband carries his valuables in his various pockets.

The backpack/daybag is primarily used for our jacket, umbrella, purchases... "suff". Because it has no bulk, we've usually been able to sort of roll it (with its contents) up, snugly, and tuck it under our arm and carry it when entering sites.

Cheers,
Vivian
PS I use a money belt as "deep storage" only when in transit between locations.
PPS... I'm in Kingston, Canada... we bought our Sea to Summit products from Trailhead, and they carry them at Atmosphere.

I made a comment to my husband about 15 years of travel with some thought to safety of a valuables. He said "Yes, we have tighten up a bit due to travel to some well known large cities with pick-pocket thieves." Can you leave valuables in an apartment? I have in an apartment in a small community and no daily cleaning. I try to get a sense of who I am renting from and the neighborhood whether it is an apartment or hotel. I do tuck them in a sock or folded in a pant, better yet in a zip pocket of a clothing. Zip and lock the bag they are placed in and into the closet. In the past, hotels had safes at the front desk and front desk staff could place your passport in a cubby hole near the desk.

@sl.wolf we almost always leave our valuables in the room. Secured and/or out of sight by whatever means are at our disposal. Room safes, locked in luggage, hidden in clothing... Mainly, we keep it out of sight to reduce temptation. We've never had a problem.

Also, for what it's worth... we've been mainly cruising in Europe, lately (going on 4th Med cruise this Oct). I will now take my passport with me, off the ship, when we are at a port where there may be a risk of not returning to the ship on time, and when getting to the next port might require a flight. I had a scare once -- got separated from my husband in Dubrovnik, and we didn't have a contingency plan. I had visions of missing the ship's departure... the next day was a "sea day", and then the following port was Malta. I later rationalized that as long as I have a credit card and my passport on me, I could somehow get a flight/transportation to the next port, wherever that may be. So, I now assess that potential before I leave the ship. :-)

Cheers,
Vivian