On any outdoor outing, I've been finding it helpful to start with the boy scout 10 essentials:

1) Knife
2) Flashlight
3) FAK
4) Extra clothing
5) rain gear
6) Sun protection
7) matches and firestarters
8) trail food
9) water
10) map and compass

Expanding on a few of these, for sun protection I include a wide brimmed hat, sunscreen, sunglasses, and SPF 15 lip balm. For matches and firestarter I include UCO stormproof matches (with stirker of course) and a bic lighter inside a waterproof case. I stuff any leftover space with tinder tabs and some cardboard. Because I am diabetic, my version of trail food becomes heavy snacks, usually a bag of beef jerky and several snack bars to supplement the glucose tablets I EDC. The rest of the list is, I think, pretty self explanatory.

I add to this with a PSK on my person and other gear to supplement the activity in question. In this case, for a day hike of about 10 miles, I'd consider including the following:

- Back pack stove with cooker and spork (gotta have my coffee, and could add a dehydrated meal or ramen to the food stuffs)
- hand sanitizer and/or wipes of some sort
- mini-tarp (when thinking about this and reading all the previous suggestions, I seem to recall there are ponchos that can also perform as a tarp and vice versa so I'm thinking of one piece of gear that could serve both functions although I don't have a specific suggestion)
- TP and shovel (you never know when nature is going to call and how urgent the call may be)
- cordage
- couple days worth of any critical meds
- PLB if you have one, or your cell phone if you're assured of reliable cell phone service

Note I said I'd CONSIDER adding lol. Speaking from experience, this makes for a fairly heavy pack, especially when you add about 1 and 1/2 liters worth of water weight (3 1/2 liter water bottles, i'd guess your built in camelback bladder has a similar capacity?). YMMV