Choose the Right Room
Ideally, a guest room is a bedroom with a bathroom attached. It should be inconspicuously placed, so that guests don't need to cross the busiest parts of the house to get to it; for privacy, it shouldn't be next to a child's room.
In a strange room, comfort and space are more soothing than a clutter of unfamiliar things. On a bedside table, place a single flower bloom in a simple glass, a nice clock, and a selection of books suited to your guests' tastes.
Outfit the Bed
Make up a double bed with four ample sleeping pillows -- two medium or firm, and two soft -- as well as two smaller pillows to prop up the head when reading. Use cotton or linen sheets, starched and ironed for hotel crispness. Provide both light and heavy blankets, as well as a lightweight throw for afternoon naps.
Closets and Drawers
Make sure there is adequate closet and drawer space. Supply a variety of hangers -- at least a dozen good wooden or metal ones -- that will hold trousers and jackets, flimsy dresses, and heavy coats. And make certain there is a full-length mirror.
If the bathroom is shared, clear space in it for guests' toiletries. Stock it with new toothbrushes and toothpaste, a plush robe, and a supply of clean cotton towels (two large bath towels, two face towels, and a washcloth) for each guest. Supply a few luxuries that one might not find at home: a beautiful soap, an unusual cream, a special shampoo, or a small bottle of perfume or cologne. If your guest has allergies, provide a hypoallergenic soap and moisturizer.
After you've provided the basic necessities, consider some of these extra touches to make guests feel at home.
Space permitting, set up a comfortable chair or settee with a pillow and throw, an adjacent table, and a good adjustable-brightness lamp. Assemble a small personal library, including some magazines and a daily newspaper (useful for local listings like concerts). A radio for morning news and quiet evening listening is a thoughtful addition; you might also include a portable CD player with a selection of music.
Provide a small desk or a cleared tabletop, and stock it with pens and paper, note cards, envelopes, and stamps. Compile a list of some favorite local places -- restaurants, cafes, museums, antiques shops, movie theaters -- and provide timetables, if appropriate, for buses, ferries, or trains. A telephone in the room is a convenience, but not a necessity.
If the room has wood floors, place a small rug beside the bed. For visitors during the coldest months, think of providing a hot-water bottle in a soft slipcase -- a soothing amenity your guests won't have expected to find.
Food and Drink
Consider your guests' food preferences: If one is a vegetarian, or allergic to fish or dairy products, be sure your menus include options and that your pantry and refrigerator are appropriately stocked. Show guests where to find snacks, drinking glasses, and utensils, and encourage them to help themselves. Leave a pitcher of spring water and a glass on the bedside table in the guest room.