I grew up on the poor side, and have been a military wife most of my adult life, so I'm a pro at being a guest and hostess =)
As a hostess, I always provide basic bedding, one towel and one washcloth, and then make sure the guest knows where to find more if it's needed (like an extra blanket, pillow, or towel). I ask if they would like to borrow any soap, shampoo, etc., or if they forgot something that they might need replaced. I always provide a fan (would be a space heater if I lived somewhere cold LOL), and ask if an extra blanket over the window is needed to make the room darker. (I'm super sensitive to light, sound, and temperature, so I know how hard it can be to sleep somewhere you are uncomfortable...and if you can't sleep, you can't enjoy your visit!) As far as food, I show them where the cups are, show them what drinks I have to offer and tell them to help themselves. Food I provide depends on who's staying with me and how long they're staying. In general, I try to make sure I have a variety of sandwich-making stuff (different meats, cheese, etc.), and fruits and veges, and also canned soups, cereal, oatmeal, frozen waffles, etc. That way my guests can help themselves to breakfast and has access to anything needed for a quick snack. (As a guest, I know how hard it can be when you're jet-lagged 3 hrs and you really can't wait until dinner!) I also make sure there is Kleenex and extra toilet paper for my guests.
As a guest, I ask where things are instead of rummaging around in drawers or cabinets. Even if I'm told to help myself in the kitchen, if I want to eat/drink something that isn't a common thing or in ample supply, I always ask if it's ok! (something other than fruit, veges, sandwich, a drink that there's only a few of left, etc.) I use the microwave without asking, but not the stove/oven. I ask where to put dirty laundry, make my bed every day (even though I never do at home), keep my belongings neat and tidy, put my own dirty laundry in a plastic bag in my suitcase, buy any groceries that I use up when I'm there (or anything special I want while I'm there, like a certain kind of soda). I always offer to help prepare meals and clean up after them. If I'm staying more than a few days, I offer to vacuum, sweep, or clean the bathroom. If I make a mess, I clean it up. If I use dishes, I wash them or rinse them and put them in the sink. I basically try to make as little extra work for the host as possible!