A national park centered around the river from which it takes its name, Krka is comprised of a handful of excellent waterfalls, easy to moderate forest hikes, and a little preserved cultural history. Enter at Skradun and pay the entrance fee (approximately 100 kuna/$18), and enjoy the attractions. Skradinski buk is most frequented by visitors, a tiered section of the river fed by a couple of falls and a rocky pool for swimming; it looks like something out of a Hollywood movie version of paradise. Roski Slap is another group of cascades near Miljevci, which is reached most easily by a boat service operated by the park.
On the cultural side, Visovac is an attraction that shouldn't be missed during your time at Krka. It is a small island in the middle of the river that is home to a 14th century monastery and little else (there is literally room for nothing more). There are also plenty of ruins on the banks of the Krka, including Necven, Bogocin, and the remains of one of Croatia's greatest fortresses, Kljucica. If you've got an extra day on your hands and feel like getting out of the city and don't feel like exploring the country's plentiful limestone massifs, this is an excellent alternative.
Note:The park is well-maintained and easy to navigate, as is often the case with Croatia's national parks.