Paint the walls your favorite color.
Lay vinyl tile for the floor and add area rugs around the room. Vinyl tile allows for easy cleanup of food and drink spills. Rugs offer a soft spot to sit and can be washed as needed.
Use overstuffed furniture. Over-sized chairs, sectional sofas, bean bag chairs or a recliner offer comfortable areas to relax.
Cover windows with curtains or blinds. This gives you a sense of privacy.
Set up an entertainment unit in the room. A television, video game systems and stereo equipment will keep you and your guests happy and occupied. Add a coffee table so you'll have some place to set snacks and drinks or play board games.
Hang pop culture-related items on the walls and around the room. Depending on your personal taste, hang music, movie and/or fashion-related posters on the walls. Place knickknacks and pictures related to your interests on wall shelves around the room.