Old halogen bulbs look dim and yellow, making a car feel dated. Swapping the entire interior and exterior to LED light sets usually costs about $400 for quality parts that last years.
You can find the best deals on these kits during holiday sales or clearance events in the off-season. Since this is a weekend project you are doing yourself, you avoid paying a mechanic for labor, which is where the real savings happen.
Remember that the 0% window is a deadline, not a gift. If you have not paid off the $400 balance by the end of the 18 months, you will start paying interest on whatever amount remains.