OOOH Sunny LOVES his pack. He looks so proud with it on. He's more obediant and gets all excited when he sees it come out of the garage. Last time he carried enough food for 2 days, plus snacks and a few toys. There is freeze dried dog food you can buy if you want to go on longer trips and cut down on weight, or go with a similar formula to what you have for your dog now but go with the "high energy" version or puppy formula for higher calories in less weight. I think a large pack would be perfect Boxer Mom. They are nearly infinatly adjustable. Sunny's is medium cuz the small just looked silly on him, and was a hair too small to be comfortable. Ruff Wear puts out a good bag as does Mountain Hardware. Mountain Hardware's is the most padded of all of them but has fewer individual pockets, just two big ones, one on either side, so you have to be careful about not overloading. Just make sure you associate the bag with fun and treats, not a ton of weight and a boring time
I think your pup will have a BLAST. OH, and be careful not to overload as (duh, you know this but I'll say it anyway) it can increase injuries just like with a human. (bigger they are the harder they fall)