I have two packs that I really like, the first is better suited to overnight tripping but is also very versatile, I have used it for day hikes and it has worked out very well, it's an LL Bean waxed canvas Continental Ruck, the other is a Frost River Sojourn Pack in the standard model, it weighs only a couple of pounds, it's about 13" wide, 16" high, and 5" deep, it's a bucket type pack with a soft divider on the inside back and a medium sized zippered pocket on the inside front of the pack, it also has a slip pocket on either side of the pack, it has leather flap straps and 2-1/2" web shoulder straps, you can pack it light for day trips but will take enough gear for a weekend camp out.
Both packs are very comfortable to carry especially if the weight is kept below 25 lbs, my usual day hike load out is about 12 pounds, over night loads is dependent on weather but usually runs under 25 pounds including food a water for a couple of days.