Marmaris- avoid , horible place
Olu deniz- nice if you only want to lie on nice beach or swim all day.
Turunc - nice little resort
Fethiye - close enough to Olu Deniz, but a town, so more interesting.
Or, you could go on a turkish boat around the coast for 7, 10 or 14 days, it stops of in little coves and bays, sometimes into ports, so you can see a few different places. I've done it a few times, and enjoyed it (as adult and 14 yr old), my mum and her friends have been multiple times. If there is a big group of you, you charter the whole boat, of there is a smaller number, then you take a number of the berths, and share boat with other people/families.