ATMs are your best exchange rate, and almost all places take cc! Technically, you could go through a 12 day trip and not need any cash - slam clicker, non tipper, room service, etc. The only place where I absolutely needed specifically cash was paying for an entry visa in Nairobi, Kenya - $25 - hard cash only! My visa consisted of a set of initials with a #2 pencil!

I usually load up on a couple hundred dollars of US (shhh, don't tell Capt Kelpto), and get what I need locally from ATMs. Occasionally I will change money at a front desk/concierge, but usually only because I'm lazy and only in small amounts, like a twenty. In NRT there is a bill changer in the lobby to convert US$ to Yen.

I never convert local back to US, some guys do, since I expect to be back again. Some guys keep baggies of velcro pouches for differnt currencies/countries.