Yeah, I know what you mean. I have a pack like that that I always have ready to go for quick evening hikes and that sort of thing. You want it to always have the basics, with room to toss in mission-specific items (Frisbee, wine bottle, blanket).
Spec-wise, you want a pack with about 1,200 to 1,600 cubic inches of capacity. Extra pockets for storage, and lots of straps and tie-downs are a plus. I am neutral on the hydration sleeve. I think it much easier to stuff a water bottle into a side pocket.
My own weekend pack is a Gregory Z22 ($108, but check REI for a closeout price). Nice little pack, with 1,312 cubic inches of space in medium, excellent suspension, packs easily, has lots of little pockets and pouches and tie-ons for extra stuff, and has two hydration ports.
Another good, small, do-it-all pack is Ospreys Talon 22 ($99). About 1,200 cubic inches, and like the Z22 it has lots of cubbies and pouches for stashing stuff (such as cell phone pocket in shoulder straps).
For something a bit larger, but also more affordable, take a look at REIs Flash 30 ($79). It has 1,800 inches of capacity, and is still compact, but with a bit more packing flexibility than a smaller pack. Nice clean design with just enough outside pockets and bungee cords, and a full-cover top lid so you can really jam stuff in there and yank the strap tightly, if need be. Well made, too.
I will say, none of these packs is water-tight. Its just too expensive in terms of materials and construction to hit that bar. A small rain cover from REI is $20thats a cost-effective solution.
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