From the port of Patras to the island of Kythira
By car my preferred route is from Patras to Gythio or Neapoli, the highway to Corinth and from there to Tripoli. Leave the highway at Tripoli and take the E961, over the mountains, to Sparta and Gythio or Neapoli.
Tolls are due on the highway!
The alternative is to continue the journey on the E65, turning at Megalopoli onto the new section A71 to Sparta. This route is 22 km further, but much faster.
The route on the E961, over the mountains to Sparta, is more scenic!
If you have enough time, you shouldn’t miss out on some of the sights in the Peloponnese. E.g. Olympia, Mystras, the Cave of Diros and Monemvasia!