There are a few options for diving and off-shore snorkelling here, but you'll be much happier heading northeast to Amed to enjoy these activities. Instead, consider taking a hike in the attractive hinterland.
Many local hotels rent snorkel sets. For the best snorkelling, take a boat to offshore sites or to Gili Mimpang (a one-hour boat trip should cost about 100,000Rp for up to three people).
There is attractive countryside dotted with several traditional villages inland from Candidasa. If you walk along the coastline from Candidasa towards Amlapura, a trail climbs up over the headland, with fine views over the rocky islets off the coast and a good swathe of the region. Beyond this headland there's a long sweep of wide, exposed black-sand beach.
Gili Tepekong, which has a series of coral heads at the top of a sheer drop-off, is perhaps the best local dive site. It offers the chance to see lots of fish, including some larger marine life, but it's recommended for experienced divers only. Most hotels can arrange dive trips.