Or, of course, you could go by public transport: very cheap, and not as bad as you think.
Kalimera, yes the transfer with the KTEL bus from Heraklion via Rethymnon to Plakias is also very easy and comfortable and cheap. They are modern buses.
The journey time from Heraklion to Rethymnon is 1.5 hours and the bus runs every hour from Heraklion to Rethymnon. Price 9? (2024).
From Rethymnon to Plakias the journey time is 45 minutes. Price 5? (2024).
If you arrive in Heraklion in the late afternoon/early evening, you often won't get the last bus in Rethymnon to Plakias - you must check this beforehand. I then often stay overnight in Heraklion or Rethymnon.
If you still have time in Rethymnon, it is worth taking a look at the Fortezza or visiting the farmers' market. The Venetian Rimondi Fountain from 1626 and the two mosques, Nerantze Mosque and Kara Musa Pasha Mosque, are also worth a visit.
The Fortezza is also definitely worth a visit (?5 per person, April 2024):
https://www.cretanbeaches.com/en/history-of-crete/forts-and-castles-on-crete/fortress-fortezza-in-rethymnonThe weekly markets on Saturday at the bus station, on Thursday in the large car park on Dimitrakaki Street (next to Dimotik?s K?pos Park) and on Monday in the car park on Kosti Palama/Xilouri Street are also nice.
Best regards from Pakias, kv