There are several churches in Granada to visit, but the Iglesia de la Merced (not far from Central Park) is noted by many as the city's most beautiful. The church on this site dates from the early 16th century, although it has been damaged twice (to the point of nearly-complete destruction once) and rebuilt, most recently in the 1860s. The present look of the interior is from the most recent reconstruction, while the Baroque facade was added in the 1780s.
Anyone can enjoy a stroll through La Merced church, but the best reason to go is to climb the church's lone bell tower for commanding views of the city center. You will be able to explore the church itself for free, but there is a small admission charge to climb up into the tower. Note that you may need to find a caretaker to gain access to the tower stairs, which are generally open from 10am-6pm on most days, and those who are claustrophobic should be aware it's a narrow spiral staircase to the top.