Painting is the easiest. Stick with light colors and paint the ceiling white. Remember not all the walls have to be the same color. Highlight one wall to set it off from the others or to draw attention to that area of the basement.
Add lighting, either track lighting which can be used to highlight specific areas or wall hangings or pictures, or can lighting for more general area light. Basements are generally the darkest room in the home so more light is needed than is usually installed in ground level or higher floors.
Have the flooring redone or tile it yourself. Carpeting is nice and much warmer, but if it gets wet it may have to be replaced. Use area rugs for added warmth and to set off different sections of the room.
Add baseboard heating with it’s own thermostat for controlling the temperature.
Remove the door at the top or bottom of the stairwell and open the stairwell up to the basement as much as possible. This will make the basement seem more a part of the rest of the house.
Add a small cooking area. Even without a water source, a counter space with a microwave and an under counter refrigerator saves a lot of trips upstairs.