Marmaris is a good base for the south-western Turkish coast. The fishing village of Dalyan is ninety minutes east, with mud baths, ancient Lycian rock tombs cut into the cliffs above the river, and the long Iztuzu loggerhead-turtle beach at the river mouth. The Datça peninsula runs west of the bay — slower, quieter, and at its far end the Hellenistic ruins of ancient Knidos on the headland.
For ambition, both Ephesus and Pamukkale are doable as long single-day trips by organised coach, leaving at dawn and back well after dark. They are easier as overnighters but possible from Marmaris if you are short on time.
By sea, the Greek island of Rhodes is fifty minutes by hydrofoil — leave at 9 am, back by 6 pm, and walk the medieval walled town in between.