A lot of the portable photo printers only print out in the small 2x3 size, which isn't ideal if you want to frame them to display. If you want larger prints, like the standard 4x6 size, then the Kodak Dock Instant Photo Printer is a good option.
While it is not as compact and even a bit bulky, it's worth it for the 4x6 printing capability. The Dock connects directly to your iPhone via an adapter (and charges it while printing), or you can connect via USB cable or Wi-Fi. The Kodak Printer Dock app lets you put stickers, filters, card templates, and more on your photos for extra fun.