The region of Marmaris offer many nice day excursions. Travelling across green mountains on narrow roads overlooking the ocean, the journeys are about as enjoyable as reaching the actual destinations. Many guide books call this area one of Turkey's most beautiful, but in my view that is an exaggeration. There are two peninuslas to travel here, the Reşadiye and the Hisarönü. The latter is smaller, but because of the mountainous landscape, it takes about as long to travel.
The journey of Reşadiye, out to Datça and Knidos is quite something. Datça comes quite boring, but the nearby old village is a pearl. Knidos, the ancient Greek city, has a specatacular setting, and is rewarding by itself.
Almost immediately after you leave Marmaris, you come to Içmeler, which is another tourist machine, smaller but more elegant, and a very nice alternative. A third resort is Turunç, even smaller than Içmeler, making it an excellent options for those who want almost what Marmaris has to offer, but with far less people.
Hisarönü offers mainly three destinations, Orhaniye, Selimiye and Bozborun. Orhaniye is a weird little place, focusing on water sports. Selimiye and Bozborun are boat builder villages, very, very quiet, so they represent radical alternatives to Marmaris.
Travelling can be done with public transportation, but then it is hard to stop by every place you wish. Best is to choose one, and connect directly to and from Marmaris. Another option, deservedly popular, is to visit several of these places as part of a cruise. Especially from Marmaris, there are plenty of options to choose from.