We've been using REI Flash 18 day packs for about 6 or 7 years and like them very much. 18 liters is a good size to carry a jacket and other odds and ends during the day. The empty pack weighs 9 oz. The shoulder straps are easily adjustable and very comfortable with the small loads the pack is designed for. The top closes with a drawstring rather than a zipper to reduce weight. The top-loading configuration with the drawstring would make it difficult for a thief to get into it while it's being worn. My wife is still using her original one. The waterproof coating on the fabric on mine started to de-laminate so I just bought a new one a few months ago.
You could look at Rick's Civita Day Pack.
I use a Pacsafe Vibe 325 sling pack, it's easy to take on and off and has a low profile so you don't have to worry about knocking into people or objects in a store. The pack has great locking features that keeps it safe from pick-pockets and is very light weight.
We have been using Eddie Bauer's "Stowaway Packable 20 L Daypack" for several years and really like them. They fold up into themselves for storage when not using them and weigh very little. They are on sale right now on the Eddie Bauer website for half off, bringing the price to $15. The larger 30 L is also on sale for $20.
I just noticed in the description that they are supposed to be "spot clean" only, but I have run them through the gentle cycle in cold water and let air dry after every trip and they still look and function as good as ever.
I agree with Kent - try the Rick Steves Civita pack. I love mine and have been using them on all my tours. It's my "purse" on the airplane. Holds a lot or a little. I like the two pockets on the front. One fits my iPad mini perfectly. It makes a good pillow when you stuff your sweater in it. At the end of my trip, I just hand wash it in the sink and it's good to go for next time.
I am a fan of packs like the Eagle Creek wayfinder. I am short and the mini would be fine for me. I would get my husband the 20 liter version. I am still using my Merrell pack that is no longer available (too bad) which is similar.
I really look for padding in straps and the back and some compartment organization. A storm flap over zippers is helpful. (I have been caught in down pours.). I don't need a computer pack.
Torso Length matters to me as well for weight distribution and fit.
12-20 liters is a good daypack size.
Use criteria that makes sense for you when seeking out a pack.
My middle-school daughter only cares about looks, "bells and whistles," and colors. She doesn't care about quality, price, or fit. To each her own.
(I'm not going to fight with her. She rarely takes my advice. LOL.)
Tom Bihn has a number of different backpack sizes and colors. I love my Brain Bag for longer day hikes where I'm hauling around a lot of food and camera gear. Not cheap, but very well made
I have and like a Riut Crush. Just over 8 oz, with zippers that rest against your back. Wait for a sale to mitigate shipping costs.
Also have and like Patagonia travel tote. Expensive but can be used as a tote or backpack. Switch to tote mode for museums.
I prefer to use my pockets over carrying anything, but when I do carry a day pack it's the Eddie Bauer stowaway 20L. Super light and zips into itself to form a small packet when not in use. It's probably not suitable for heavier loads but you shouldn't be carrying around all that stuff anyway.
I like travel on - very reasonably priced..
I use the Timbuk2 Uptown laptop/travel pack, and quite like it.
I even had a big bottle of hand lotion blow up in it once (don't ask!) but it dried out and handled that crisis well. It has well thought out compartments and pockets so I can easily sort what I need, as well as plenty of pockets and compartments to safely seal certain things inside (a padded laptop compartment that doesn't encroach on the rest of the bag but is easy for me to get to while difficult for a thief to get to, pockets that take my passport or wallet well), a handy pocket on the shoulder strap for my cellphone, and other neat features. Very lightweight and durable, but as you likely know once you get a back designed to accommodate a laptop (with the padding) you can often trade off on other aspects (can't roll it up, for instance).
I've been using either a RS Ravenna or Veloce bag lately. The Civita is a tad too small for me, and doesn't have a trolley strap for attaching to a rolling bag, which I really like. I've also used the packable Eddie Bauer backpacks, which are easily compressed for a suitcase when you aren't using them.
Of the two RS daypacks I use, the Veloce is slightly better in that it can be used as a shoulder bag as well, and expands for larger loads. It's a bit pricey compared to comparable RS products, but its versatility makes it my preferred daypack. It slides over the handle of a rolling bag so is a great personal item for a flight, too.