Your American crackers/pilot biscuits would be similar to 'cream crackers' in the UK IIRC.
Ryvita is a hard rye biscuit
http://www.ryvita.co.uk/products many are available in the States an American chum buys them.
Pumpernickel is German rye bread, sliced thin, lasts for ages
http://en.wikipedia.org/wiki/Pumpernickel Its like Marmite, you either love it or hate it, I love it.
Pita (or Pitta depending on your spelling) bread is just a simple but thicker flat bread which really came over from the middle east. You can buy a packet of six for 50p/75cUS which unopened last for months and will last several days after opening without refrigeration. Heated over coals or a stove they 'puff up' and you split them and fill with filling of choice kebab style.
Mexican style 'wraps' have really taken off in the UK the last few years, again you can get long life packs as I'm sure you can over there with you.