You are likely to find locally organised excursions though travel agents / tourist offices when you are here, for example you can go to Elounda and take the boat to Spinalonga. For Cretan cooking you can look at www.cookingincrete.com
Someone else here on the board may know of activities going on, and the best churches in those areas ?
I wouldn't worry too much about the dress code at the Easter service - everyone tends to be dressed up like for a night on the town and just about anything goes.. really! even in village churches...it's like a fashion parade. Forget those old images of demurely dressed young women.