Below the the island that Tintagel Castle perches on is a small sheltered pebble beach which is good spot for a swim or snorkel (there are kelp beds with plenty of marine life and grey seals are often around).
On the left side of the beach, Merlin's Cave goes all the way beneath the island and out the other side. There is a large flat rock just out to the right from the beach which you can climb down to from the coast path which is a good spot to fish from for mackerel at high tide.
You can walk along the coast path from Trebarwith Strand to Tintagel Haven (about a mile) so you can do a 4-5 mile circular walk starting at Park Farm and coming back through Tintagel (there is a circular route mapped out on our walks page). Alternatively if you drive, there are a couple of possibilities for parking: