Xanthos is great to do on your own, we went in May - very quiet! Get a decent guide book before you go there aren't any available there, in May there was only a guy collecting entrace money. Also recommend decent shoes it's a huge mountainous place, quite rocky under foot and takes ages to get around. Would recommend Tlos and Patara beach as well not to far away and worth a visit. For futher afield Ephusus is a must! However you may want to consider a organised trip for this one - it's quite a drive!