This is the type of thing that would be great.... Would love a non bias review.
http://www.amazon.com/gp/product/B009V5X1CE/ref=ox_sc_act_title_1?ie=UTF8&psc=1&smid=A2KUZVNQ9LP7N9
I have three kids who are absolutely addicted to their electronics, and I've now bought six of the RAVpower battteries. All of mine are newer (13,000 or 14,000 capacity) but I've gotten excellent, reliable service out of them.
They seem to come up with a newer model every few months with slightly larger capacity, but then the "old" one dips below $30 on Amazon which is where I buy them. You can easily run an iPhone for a week with very heavy use, or an iPad for a couple of days. By "heavy use" I mean hours at a time while on an airplane or in an airport watching videos or gaming 16+ hours a day.
Highly recommended