Here is some helpful information on flying to Crete and getting to Rethymno.
Flight Information
All flights to Crete for May 2017 have now been released (this includes RyanAir and EasyJet).
Crete has two cities with international airports, these are...
Heraklion (HER) and Chania (CHQ). Both are approximately the same distance to Rethymno (about an hour's drive)...
There are regular direct flights throughout May to both airports from Manchester and London. When searching for flights on Skyscanner search to/from both Airports.
In regards to flight costs and as a guide - we flew direct from Manchester on May 14th to Chania airport using Ryanair and then returned direct a week later from Heraklion (Thomson) for a total cost of £220 each.
Airport Transfers to Rethymno
There are shared shuttle options that work out to be fairly cost effective ie £30 return to/from the airports. See here...
http://www.airporttransfers.co.uk/to/rethymnon/
From Chania to Rethymno see here for more useful information...
https://www.tripadvisor.co.uk/ShowTopic-g189415-i714-k6074628-Chania_Airport_to_Rethymnon-Chania_Town_Chania_Prefecture_Crete.html
Taxis - a more expensive option but for a taxi of 4 people expect to pay around 90€ one way to/from the airports.
Car Hire
For anybody spending more than a few days in Crete and looking to explore the island a little then hiring a car is the best option. Crete is big and hiring a car is cheap.
We used Athens Car Hire and paid £20 a day for a small car. They also allow you to pickup and drop-off the car from different locations (30EUR charge). Athens car hire link is here...
http://www.athenscars-crete.gr/car-hire/
Parking in Rethymno Old Town
The old town streets are kept for pedestrians only and cars will always need to be parked away from your hotel if you stay within the area.
There is a large car park next to the Old Town that can be used for 5€ per day - make sure you purchase a day pass on arrival from the booth otherwise you are charged by the hour. See my Crete map for the exact location...
https://www.google.com/maps/d/viewer?mid=1KsyxLAb72LAmX88On3YWaJ2DQ-I&usp=sharing