I very recently went through this very same decision process. I'm close enough to Disneyland to get the local discount on season passes and have visited the park over a dozen times in the last two years. Some were day trips and some were long weekends. I'd been using a Grey Ghost Gear Stealth Operator Pack in their discontinued blue & grey color. It's 14-15L so it's on the small side, but very unobtrusive. With wife & 3 kids, it was maxed out, but if you're smart about packing it's very doable. Even when it rains here, it doesn't really downpour for any length of time like other areas of the country. Average is 2-3" of precip per month in the winter. We've gotten by just fine every time with just a cheap $1 "emergency" poncho, silkweight polypro shirt, and pair of spare socks for everyone, plus snacks and water and other little ancillary stuff. Layers that get shed after the morning starts warming up still fit.
We'll be doing a little more traveling soon so I've decided to plus up to a new slightly larger pack. I also looked for a pack that I could use for long dayhikes or short overnighters, but honestly anything larger than 30L just seemed too big for my main purposes. I'd rather have the right pack for the right purpose. I ended up deciding on the Deuter Speed Lite 20 pack. Not too big, not too small, just right. The compression straps can be used on each side or wrapped all the way around and connected to each other to hold bigger items on the front of the pack. Shock cord can be woven through the the loops on the front of the pack to hold jackets etc. With the straps and bungee cord, I can strap real rain jackets for everyone to the outside of the pack if needed, and still have all the other stuff I usually take plus a bit of extra room.
If I wanted something bigger for long day hikes that could turn into an overnight, I'd pick up a Deuter ACT Trail 30. I really dig the top load and panel loading feature to access top, bottom, or middle of the pack as needed. I'll probably end up grabbing one sometime in the spring or summer. If the REI Coop Trail 40 just had compression straps, it would be a winner, but not being able to shrink the bag down to stabilize it with less-than-full loads, or strap stuff to the outside, makes me pass on it.
I'm a fan of Deuter bags. Good quality for the price, and usually have the features I'm looking for. They're not as popular in the US, but they're all over Europe. While not a deciding factor, I'd rather be mistaken for a European tourist than an American in most cases where people watch for and notice such things.
After looking at so many packs, I think it would be hard to go wrong with any of the quality brands mentioned, with one caveat. If it's got any sort of suspension system, do your absolute best to try it on in person before buying. Some people find certain packs super comfortable while others can find them physically painful if they don't fit their body size or type. This is one of the reasons I really like REI, even though they're generally more expensive. They have a killer return policy. No questions asked for up to a year on most gear. The employees that I've talked to actually encouraged buying stuff, trying it for a while, and returning it if it didn't work perfectly for you needs.