I have a deuter 35 which I find very comfortable, and which does for that kind of trip. It's a bit heavy though. I also have and Osprey talon 33, when none of the children have pinched it, but I don't find that so comfortable if it's full. For a longer walk I have taken a lowe alpine 45/55, which again is very comfortable but probably bigger than you need. I find it handy to have a bit of spare space though, it's easier to find stuff and there's plenty room for food etc.
The best thing to do is probably put everything you want to take together, go to an outdoor shop and see how big a sack you need to put it in. |Then try on LOTS, with a bit of weight in, and see which you find most comfortable. What suits one person doesn't always suit another.