Not sure if you have coffee maker, fridge, etc, but Premium Spirits has great coffee (fresh ground). Sail Away Cafe used to sell their coffee retail as well. Both just south of you. Lots of little groceries around - I prefer to patronize Greenhouse (which is on middle street) as they have the best selection and variety.
If picky or OCD like I am, you may want to consider bringing some of your favorite cereal / snack foods with as you can't always find specific items (for example, I haven't found grape nuts in over a year now - even on the mainland, but luckily I have friends who have kept me supplied).
Cash is probably easiest for groceries as everyone accepts US bills.
Hope you have a wonderful trip