Here's what I'd carry in the Synapse 19 (original!) for a long day hike in the summer, assuming I'm carrying just for myself and no one else. Note that my packing list changes considerably based on the season. I'm not claiming to be the most expertly prepared hiker or anything like that... this is just the stuff I've found that works for me. My 7D w/a 50mm lens can fit in the Synapse with this packing list, but I tend towards the Smart Alec if I'm going to take my camera. In the Shop Bag to the left is what I'd bring to leave in the car (more on that later).
Clear Organizer Wallet (a paired down version of its usual contents)
CocoHydro evaporated coconut water packet
Katadyn Hiker Pro water filter in a Size 3 Travel Stuff Sack (you could squeeze the water filter into a Size 2 Travel Stuff Sack, but personally I'd rather have the extra room that a Size 3 gives so that I can shove the filter back in the stuff sack if I'm in a hurry, like if there's mosquitoes)
First Aid Kit
Extra first aid stuff: blister things, extra Benedryl (for wasp stings for humans and dogs)
Extra Head Lamp (something I learned from Tom: watch for sales on cheap headlamps at places like Big 5 Sporting Goods, buy several, and always take an extra with you on hikes in case you run into fellow hikers coming out late who didn't plan ahead with their own headlamp.)
3D Mesh Organizer Cube in Wasabi Dyneema w/14" Key Strap in Ultraviolet
Emergency firestarter kit
Extra Battery for my flashlight (see below)
Peanut Butter/Cocoa/Date/Coconut energy balls (Unless I'm actually planning to eat dinner on a hike, I don't like to take a lot of food, or particularly bulky food. I'd rather have enough to get through the hike and then eat way too much Mexican food later when we're back. That said, IMHO, it's always important to take some calorically dense food, just in case.)
Synapse 19 (original!)
Keys clipped to the included 8" Key Strap
Skeletool tethered to a 14" Utility Strap (obviously useful in many situations, including hiking with dogs in porcupine country… ouch)
HDS EDC Executive 120 tethered to an 8" Utility Strap
Buff, merino wool (this and the wind/sun shell stuff into the Size 1 Travel Stuff Sack in Ultraviolet)
It gets cold/I get stuck outside warm clothing: wool tights, synthetic/fleece top with hood, wool gloves (compresses very nicely into a Size 2 Travel Stuff Sack)
Here's everything kitted up and ready to be packed:
Even with all of that, there's still room for more stuff...
And here's what the Synapse 19 (again, note that it's the Original) looks like packed with all of that stuff.
In the Shop Bag is a bunch of stuff I'm likely to toss together to keep in the car. Maybe I can get back to the car, but can't get back to town/home/whatever. Or, maybe I run into people who weren't as prepared as I was and might need some of this stuff. And some of it is just stuff that's nice to have at the end of a hike.
Gallon of water
Chacos (nice to wear after a day of hiking in boots)
Cotton hooded sweatshirt (Yeah, I know, cotton kills (Kenny?)
Extra pair of socks
Bigger First Aid Kit
1000+ calories of almond butter, honey, sea salt, coconut in a glass jar
Guardian Light w/lanyard
MICROspike crampons (I might bring those on the hike depending on what it's looking like when I get to the trailhead..)
Dog food (I guess you wouldn't bring this if you're not hiking with dog(s)
Lightweight umbrella (I might bring this, again depending on what it's looking like at the trailhead..)
Another extra headlamp