I might just take 2-3 AA batteries .. rechargable with a nice 2500mAH.. which would mean this would last about 125 hours on 1 full charge.Thats about 1 charge a month...
If you decide to go the NiMH rechargable route, go for the ones with a low self discharge. Panasonic calls it R2 technology. They have a lower capacity than regular NiMHs, but when used in low drain applications like yours, will last much longer.