After getting the Sandals of Hermes and the power of fire from Prometheus, you can reach Apollo's temple in Peloponnesus.
Go to Peloponnesus
Leave Zeus's temple the way you came in, defeating the bull again, and follow the path until you are back in Arcadia. Go all the way to the right until you reach a dead end, and go into the door in the tree. You will reach Peloponnesus.
Reach Apollo's Temple
In the forest, use the fire from the Staff of Fennel to burn the spider webs that will be in your way.
Bats will attack you here, as well as large birds. The birds can be defeated with two hits from the Staff of Fennel.
From the first area of Peloponnesus, go to the right until you reach a dead end, then go through the door. Follow the path until you are in a new area of forest, where you see a spiderweb to your left. Go to the right and enter the first door. You'll be in an almost identical forest. Go to the left and enter the second door (it's to the right of a spiderweb). You'll find Apollo's temple. Talk to Apollo to get Apollo's Harp, then talk to him again to get a password. You can use the harp at Sun monuments to summon Pegasus, who will take you to a different location.