I just back from a week in Marmaris, stayed in a great hotel called the Grand Pasa.
I wasn't sure if I would like it, especially Marmaris, but I was pleasantly surprised as I thought it might be a bit like the costas etc. Plenty of good value but decent restaurants, shops etc.
It was my first time going all inclusive, and there was a great selection of food in that hotel. It was mainly families, but has a mix of couples, older people etc as well.
Marmaris is quite a large resort, but if you were looking for somewhere smaller, Ichmelar nearby has a slower vibe without being too dead.
I've been to Altinkum (in the vicinity of Gumbet) previously, and while I enjoyed it well enough at the time, I prefer the Marmaris area. Better sights, selection of shops and less hassle as well.
I've also heard good reports about Kalkan.