I just received really great answers to all my questions. However, there is one key question that I am still struggling with: where to stay for a week if we want to have a HQ and do daytrips?
We are travelling with our 2-year-old son and are looking for a small town that is centrally located between a good beach, Efes and the more Southern ruins like Dydima. We are not looking for places with big crowds...maybe smaller coastal towns...
So far we have Kusadasi, Foca and Cesme as options......but I guess Cesme is not really that central....
Thanks for your help!!!!!!!!!!!!Text

Cesme might not be the most central but due to its great highway location the most accessible town on the weat coast ,it is the one with most Turkish population in the summer (less tourists) and has great beaches.
There is ofcourse the proper Çesme town which is full of Bed and Breakfast places,pick your choice..the town has an obligatory castle,a sea side promenade, a bazaar, lots of restaurants,etc.. but it is not a great place to swim.Just outside the Çesme town,before you even enter the Çesme town there is another district of Çesme where most of the summer houses are.That part is called Ilica.You will prefer to stay there because it is right on the 4 km long sandy beach.The below link may give you some ideas.
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Kusadasi is very crowded. When we passed it by car we were shocked by all the lights and the crowd.. :) It looked like a big place, and it's something that we wanted to get away from for the holiday. :)
We stayed in Gokova, it's 30 minutes or so away from Marmaris. It is located in a gulf of Akyaka (another name for the village is Akyaka), so the village is not right next to the main road. It has its advantages - not many people, pretty quiet place, and disadvantages - you have to take a taxi or a minibus to go to the main road. It is not very expensive, it takes only about 10 minutes to go up, but still it is an extra expense. It has a few good restaurants, bars, the gulf is nice and you can take walks in the neighboring hills, a lot of fresh air. They have some water sport stuff for rental, but not very cheap, but I guess it's never very cheap. There is a lot of accomodation, I can recommend a nice place if you are interested. But maybe it's not a good option if you need to go to some excursions every day.
Another option where we didn't stay, but we stopped there for lunch - it's Pamucak. It's closer to Kusadasi, but the place itself is a small place with hotels. There are hotels right on a huge sand beach. The place looked very nice and I guess it is not far from the main road.

I went to Foca 2-3 years ago, stayed in the old town area and thought the place was extremely charming, love the place, but we had a car and the place may not offer you the mobility you are hoping for without one. May make your decision to rule out Foca easier if you want to find tours with other tourists, you probably won't get it there, but lovely locals.
Next to Efes is another charming small town of Sirince, but that probably won't offer you the beaches you want. Make a stop there and taste some of the local wine on your trip to Efes.
For some reason I don't like Kusadasi, but have absolutely no reason why.
GTI makes a good arguement for Cesme in my opinion.
You could also look for places to stay in Didim. Keep in mind you may come across many Brits as the new locals, as they've taken up lodgings there now.

Selcuk is the obvious choice. You can stroll to Ephesus, dolmus to the beaches between there and Kusadasi (no big deal), or take the coastal road to Cesme in a couple of pleasant hours. It's very easy to get to Priene, Miletus and Didyma on an inclusive day trip, and you can also access Afrodisias, and even Pamukkale. There's lots of good accommodation, with the Hotel Bella a very good option. The town itself is very attractive, with a piece of a Seven Wonder, a scenic kale, a beautiful ruined basilica, a couple of classic camii, and a nice town square with excellent dining options.