The key to creating a home office in your guest room is to make all the furniture and accessories multipurpose. Instead of a guest bed, incorporate a sleeper sofa, which can be used as a couch when the room is serving as an office. A day bed is an even better choice because it takes up less space and doesn't need to be opened or closed. If you have the money to spend, you can even buy modular furniture made specifically for your purposes. For example, you can get oversized reading chairs that convert into beds or a Murphy bed that folds up inside a closet when not in use.
Instead of a coffee table you can use a trunk that can also serve as storage for bedding and pillows or for your office files. Another multipurpose option is an end table, which can be used as a nightstand for guests. Hide unsightly computer equipment from guests in an armoire made for computer storage, or customize one you already own by drilling holes in the back for the cords.
Table or floor lamps are useful for office lighting and can also serve as reading lamps for your guests. You can make the room homier to guests with an alarm clock and a small coffeemaker, which you're sure to appreciate in your office as well. Feel free to line any bare walls with bookshelves. Along with your reference books and office supplies, include some books and magazines for your guests to enjoy.
When decorating your home office/guest room, you can choose from a palette of soft, neutral colors, which will be conducive to relaxation, or you might prefer warmer tones, which will prove inviting to your guests and energizing and motivating to you during your work day. Make sure to coordinate the bed linens and artwork in the room with the color scheme of your choice.