A really great starting point is Arc’teryx's Altra 65 ($399). At 4,000 cubic inches, it falls in the sweet spot of pack sizes: it's not too big for an overnight but has enough capacity for a four- or five-day trip, depending on the gear requirements. I also recommend it because Arc’teryx's stuff is extremely tough, and the Altra, which is made from a durable, lightweight packcloth, is no exception. And the suspension system is great.
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