Marmaris is a 3 hour journey, so six hours in all in a coach. Kalkan is a nice little place we often go there. There are very steep lanes down to the harbour and beach, so if you have any walking difficulties it may not be the best place for you. However, if you don't wear some flat shoes with backs on them and you will be fine. Some nice restaurants down at the harbour and lots of little shops. Catch a bus there from Fethiye.
Kevin's idea is good, you can book a coach to take you for the day to Tlos, Yakapark and Sakilkent
Also you could consider a day trip to Rhodes.