There are actually several bridges in Putrajaya, but the most popular and iconic is the Jambatan Seri Wawasan. This is the "sail" bridge, and has a great view of the mosques and lake. If you come on a Tuesday or Friday night, there's a night market roughly 300 meters away (head towards the direction of the Putrajaya International Convention Centre aka PICC).
Other noteworthy bridges (if you have time) is Jambatan Seri Gemilang (heading towards the PICC, design resembles a bridge in France) and Jambatan Seri Saujana. The former has a park nearby and the Putrajaya dam. The latter offers a nice view of Jambatan Seri Wawasan and is generally quieter.
Do note that these bridges don't actually have proper parking bays or facilities etc.