Jackery, despite the silly name, makes many popular and well-reviewed USB battery packs, and one of its newer models is on sale today.
The Jackery Bolt (pictured above) is a little expensive at $22 for 6,000mAh, but it includes a built-in Lightning and microUSB cable, so you won’t have to worry about carrying a spare. If you’re the forgetful type, that’s well worth the price premium.