Hi,
Cant really comment on Rethymnon or the studios you have chosen as I have only passed through the resort, all reports I have had though are positive.
As I understand it the most popular beach is the central one. The sea temperatures in Crete are a degree cooler than the Ionian Islands for instance but still comfortable and refreshing. Generally it is still very hot and warm enough at night not to need a jacket/jumper/fleece etc, many will say its an ideal time to be there as the nights are much more comfortable than July and August and you can at least sleep in comfort.
One thing I have noticed is that eating out etc in Crete seems considerably cheaper than many other places. Last year in Georgioupolis and Gouves I was paying far less than I was 2 years previous in Santorini and Majorca in 2004! Probably because of the size of Crete, the fact that it produces far more than the smaller islands and doesnt need to import as much may be factors. In comparison to other Greek islands, Spain etc, the Euro does seem to go further!
If you are Self Catering stock up on the beer, surprisingly good local wine and spirits, a bottled beer at the Supermarket is about 70/80 cents, the same in a bar maybe as much as 4.50 Euro.
Car hire anywhere on the North coast is always a good idea, Crete has a very good North coastal link, the only hazards which are really different to here (UK) are the unqualified nutters (tourists) on hired quads, scooters, mopeds and motor cycles, stay well back from these irresponsible, negligent, ignorant and stupid people who really couldnt give a proverbial for the safety of others ................................, er, maybe that is a bit harsh. Because of their incredible stupidity they really know no better and perhaps deserve pity, they are after all the victims of little or no education, poor parenting, victims of the Life Guards absence in the gene pool, the "I know best" and "am I bovvered" approach fosterered in the UK and of course the CHAV culture! Nothing worse than seeing a scrawny, shirtless dimwit in Union Jack shorts tearing around the roads with a belly full of drink and a head full of space oblivious to the consequencies of his/her actions, (oops, sorry, Craven on a soap box), but you get my drift!
There is a useful thread on car hire on these Boards with recommendations for booking online etc.
Day trips, there are many, depends entirely on what you want and range from walking the Gorge (need good footwear) to a bit of history to lazing on a boat to a visit to the lovely Chania. Try doing your own thing, the mountain villages if you hire a car are well worth a day out. You will see a variety of trips available when walking round the resort and/or your Rep will be able to sell trips (a touch more expensive than DIY) or provide advice.
Have a lovely time.