Day Boat Trips with a Kaiki to the North eastern coast can be taken from Gouvia, Dassia or Ipsos .If you want to rent a Boat on the North East of the island, this is also possible from Agni or Agios Stefanos.
Skuba Diving and Snokkerling is best in Nissaki or Kassiopi.
For Horse Back Riding check out the Silvaland Riding Centre, close to Varipatades, North of Kalafationes. Someone can arrange for riding lessons as well.
Bike Tours could be taken at Dassia, Acharravi and Pantokratoras besides other places.