I recommend some sort of NiMH "hybrid" rechargeable, such as Rayovac Hybrid or Sanyo Eneloop (currently, Amazon has the Hybrids for a bit cheaper, but Eneloops are a bit more widely known). Of course, you'll also need a charger. If you don't already have one, Apple's own charger also comes with hybrid batteries (probably rebranded Eneloops, but we don't really know), and it's not terribly unreasonably priced, so it's not a bad way to go.