USA - there's no problem using your card to buy a sub $1 Cola from a fast food store, or similar from other places
That said you want a true exchange rate card (like my Halifax Mastercard) like there are a few available. You may just manage to get one it time for your holiday - go prepay maybe.
As said otherwise you're not really getting enough cash to make a huge amount of difference between now and then unless they suffer hyper inflation or something surely?
Cash is really useful in some situations though i.e. tipping, or if your renting a car and need to top up fuel.
Example: I found in AZ/NM/NV that the majority of pay-at-pumps didn't recognise my card, and staff didn't think there was a round around it.. as they needed the zip code it was registered to (DOUBLE EDIT - it didn't recognise UK postcode or just the numerics from it), so to save lots of faffing around with passport (ID)/pre-pay/refund on my card it was easier to do a quick mental calculation of amount of fuel required, and hand the cashier a 10, 20 etc dollar note and the pump would stop at that amount.
EDIT - I'm sure someones gonna come along and tell me the way round this next time though