Crete 5 days itinerary review

You are looking to do a lot in a very short time. The drive to Elafonissi is likely about an hour and a half provided you don't see something that causes you to stop. There are good beaches just 3km west of Chania. You can walk, drive or take the bus to these beaches. Closest is at Nea Chora, then Golden beach (Hrisi Akti) and then on to Agii Apostoli. You can do your beach walk without needing a car.
However if you want to do Elafonissi it is beautiful.

Another option however is to drive to Kissamos which is on the way to Elafonissi and take the boat excursion to Gramvossa and Balos. This excursion gives you 2 great beaches plus a 14th century pirate fortress to hike up to.

We go to Plakias every trip to Crete. Its a good choice. While the only way to get Preveli beach used to be by boat or the parking lot at the top of a cliff recently a paved road has been completed which takes you to a parking lot 5 minutes from the beach.
Here is information, To avoid the difficult climb, you can alternatively drive in the paved road leading to nearby Drimiskiano Amoudi and walk the short path that leads to Preveli in just 5 minutes.

The beach at Plakias is blue flag and is a great walk on its own right. It is framed by mountains and a sheer rock face at the far end.
Warning by the rock face bathing suits are extremely skimpy.

Here are some images of your trip
Chania https://www.flickr.com/photos/stanbr54/albums/72157632117917828

Gramovossa,Balos and Elafonissi https://www.flickr.com/photos/stanbr54/albums/72157698932695812

Plakias https://www.flickr.com/photos/stanbr54/albums/72157665045541263

Back in Plakias https://www.flickr.com/photos/stanbr54/albums/72157688831637594

@stanbr Thank you very much for the info. Your insights are very helpful for planning our trips there. Now I am contemplating changing my dates and flying 2 weeks later for better, warmer days. The price difference is not so big so I think it may be worth it. What is your opinion on this - early vs late May.

Is Kronos auto correction for Knossos? If you're interested in ancient history, it and the Archealogical Muesem are a must. The Minoan cultures were as ancient to the Ancient Greeks as the Ancient Greeks are to us today.

When we first started to go to Greece we always planned it after May 15. The weather seemed to change and was all bright sunny days. Last few years we pushed back into June. I think later is better. Having said that we are arriving Crete May 3 so that will be a test of weather patterns.

You are going to need to rent a car that allows you to pick up in one city and drop of in another. We use AutoRentals-Crete. They may be willing to meet you say at Talos square and deliver the car to you there. You can also ask them where they want the car dropped off at Heraklion. They are locally owned so are prepared to provide superior service to get your business. There are a few other locally owned companies but I have no experience with them. See Best cars, Anna cars.

Thank you for the advice. I decided to keep my dates, too much hassle to rebook everything plus everything get a little more expensive.

The day we drove to Elafonisi from Chania we stopped at at Falasarna Beach , totally different types of beaches.