You can hit up some of the best beaches on that particular part of the coast very easily on the bikes from there. Nothing particularly picturesque just wide sand and calm shallow blue seas.
If you can make it as far as Giorgopouli (sp) to the west then do as you'll be rewarded by a nice beach...
Oh and the Samaria Gorge too. A long hike if you drive and park at the Northern end or alternatively be lazy and take the boat there and meander at your leisure
Papa; best advice really would be to hire a car or bikes and get out and explore the interior of the island.
Some spectacular mountain passes and valleys which in the summer smell of all the wild herbs that grow by the roadside.
As far as excursions Rhethymnon is worth a trip - medieval...
In Santorini make sure you try their local delicacy of fresh baby squid stuffed with feta, peppers, onions, tomatoes andherbs which is wood oven caked until charred and delicious. Drool