This unisex line includes the largest pack Arc’teryx offers, at 95 liters, and is well suited for expeditions lasting longer than five days. The lid is easy to remove and can be worn as a small lumbar pack. With interchangeable hipbelt and shoulder straps, it is also the kind of bag you’ll utilize for years to come.
Pro: The lid-turned-lumbar pack has a pouch for a small hydration system.
Con: It is pricey.
arcteryx.com; $399-$449

Osprey offers two backpack series with removable day bags: Aether for men and Ariel for women. The mid-size expedition bags are super light, weighing roughly 4.9 lbs, with gender- and size-specific harness and hipbelt systems. The bags are also sleek and non-bulky. The lid detaches and converts into a lumbar pack.
Pro: Highly tailored fit options.
Con: As with many convertible lids, the waist straps that are used to make it into a lumbar pack have no cushioning.
ospreypacks.com; $239-299

The Red Cloud packs are Kelty’s biggest load haulers (up to 110 liters), and generally receive rave reviews from their owners for superior fit at an affordable price when compared to its competitors. The lid can be removed and converted into a lumbar pack, though it should be noted there's no separate water storage when used this way.
Pro: There is a women’s-specific 80-liter version.
Con: Rain cover sold separately.
kelty.com; $199-$219

These expedition packs have a small 10-liter bag clipped to their fronts. Unlike most other convertible packs, the day bags are actual backpacks with wide, padded straps. This means you’ll not only summit more comfortably, but may also be more likely to use it independently of the larger bag.
Pro: The pack is svelte even with the extra bag attached, measuring only 10 inches deep.
Con: It is heavy for its size, more than 6 pounds for a 55-liter volume, and is most frequently used for extended travel, like backpacking across Europe.
deuter.com; $239-$259

This multiday pack has a 70-liter capacity (the women’s equivalent, Juniper 50, has a roughly 50-liter capacity) and small bag clipped to its front. The day bag’s wide, padded straps (similar to Deuter’s Quantum packs) and hydration pouch make it as good for short local hikes as it is for summit pushes on longer treks.
Pro: The pack’s fabric is made from recycled plastic bottles.
Con: It doesn’t have a separated sleeping bag compartment, though the U-shaped front zipper access helps overcome this.
mountainsmith.com; $195-$200