I also have the 26L Backlight from Mindshift and I think the 18L would probably be the right size for what you want. I use the 26L at the zoo and I don't feel it sticks out too badly. I see plenty of people using ~30L daypacks, particularly those with kids like me.
Another thought is an f-stop Lotus with a medium ICU. While this is 32L and bigger than you have talked about, it looks a bit more ordinary and doesn't scream camera bag as much as most other packs, and will leave you with enough extra space for the light jacket, snacks, water bottle, bug spray, etc. I'm not fond of them as a company but the design is good. I debated on this vs the Backlight and went with the Backlight obviously. You'll need a sleeve for your tablet if it doesn't have a good case already.
Here is a medium pro (no longer made) ICU with a rough idea of the space (this is the 37L Loka, a tad bigger than the Lotus). I carry a 100-400 mounted, hood reversed and up to 3 other lenses plus filters, spare battery, remote.





