A solid set of tires is the most important upgrade you can make for your car. They handle the road differently, improve your stopping distance, and keep you steady in bad weather. When the time comes to swap them out, you are looking at roughly $1200 for a quality set.
Timing is everything when you are shopping for rubber. Look for holiday weekends when shops run rebates to move inventory. Once you have your quote, separate the act of negotiating the price from the act of paying for it.
Remember that the 0% intro APR is a deadline, not free money. If you have any balance left after the 15 months are up, the bank will start charging interest on whatever is still owed.