How can I temporarily finish a basement without actually finishing it?
You could pin up several runs of Backdrop Drapes along the walls. They come in many colors and are less than $40 for a 10' wall span. You could cut out around the windows and even hang a temporary blind. If you already have all of the electrical installed it would be easy to pull some short extension cords underneath or in between the curtains for TV's. 

Some cheap indoor/outdoor carpet for the floors would be cozy if you threw down some temporary rugs. You could use a few screws and pin it to the bottom plate to prevent too much sliding.