Curious about your thought process.
1) Why do you need a summer day hike pack that is compressible?
2) A bladder carried water closer to your back and is a lot more stable. It also keeps the water colder on the hot days.
3) 20L is huge for a day hike pack, especially in summer. Are you sure you really need all that space?
I use an Osprey Talon 11 with the hipbelt cut off for done-in-a-day trips. It carries:
Rain jacket and skirt (under bungees)
Down jacket, vest or synthetic vest
Gloves and hat
Blizzard bag (emergency bivies)
Typical misc kit (first aid, repair kit, sun screen, map, compass, headlamp, whistle, Spot)
I can also easily throw helmet, crampons and an ice axe in for technical trips. This covers everything from a few miles to 16+ hour epic off trail ridgeline traverses in cooler fall temps.
What else are you bringing on simple summer day hikes that you need 20L for? (maybe the Talon 11 is bigger than 11L though).
I wish they made a 30L identical to the 11 for 3-4 day trips. The pocket layout is perfect - not fussy at all, just what you need. Shoulder pockets for camera and a powerbar. Exterior pocket for quick-access for common use essentials. Interior pocket allows easy access to the day's food. External bungee for raingear. Open bladder pocket allows you to easily stuff your windshirt for quick on/off.