Hi pip7
Rethymno is really nice in the old town, round the fort and harbours, with plenty tavernas, shops and bars your spoilt for choice,
the sea front tavernas seem to be quite pricey compared with those up the back streets in the town, (avoid the special deals on offer, they can be poor quality and quite measly portions)
plataines is quite commercialised and is actually further along the coast from Rethymno, but as it sits on the main drag you will be able to use the public transport,
Please take a bus to Chania and visit the Harbours and museums and explore the back streets.
Santorini is an option for a day excursion, you can book the seacat in Rethymno (avoid booking with holiday rep, as you save money by booking yourself)
Only be tempted by Samaria gorge (long way) if you have stout boots and good legs, The path through the gorge is very uneven (big stones) and steep at the start. you are so intent on not slipping or tripping over and getting to the bottom before the last boat leaves, (the boat we went on left 2 couples behind) you miss most of the scenery.
I hope this is of help to you.