Defintely second taking the trip to Symi. I'd suggest getting a cab up to Rhodes Town, then getting the boat from the harbour (there's loads).
As for the trip to Turkey, I've done it a couple of times - you don't get to see much of Turkey, but if you want to go shopping in the bazaar at Marmaris then you're sorted - lots of cheap fake-designer stuff. One trip we had included lunch, and one didn't, so check what you get if you decide to go.