Kas just might be my favorite place in Turkey, it's just that great. It's pretty small compared to the other Western Med getaway spots, it's totally mellow and there are gorgeous flowers everywhere. There's also a lot to do and it makes a great base for day trips -- you can go scuba diving or sailing, visit ancient ruins at Myra, Tlos, Xanthos or Letoon, go paragliding, go to a real sandy beach at Patara, visit Greece for an afternoon, or go kayaking over the sunken ruins of Kekova. There's also now a large marina, if you've got the yacht.
Of course, it's also close to my heart because Kas is where Cagatay and I met, during a scuba-diving trip. That first day, he extended a friendly invitation to go to dinner at his uncle's restaurant, Ikbal -- which might actually be the nicest restaurant in Kas. (Seriously, I'm not just saying that because we're family now -- the food is incredible and the setting charming. Look at Trip Advisor -- it's ranked #8 of 96 -- if you don't believe me.)
Another thing I really like about Kas is its energy at night. Because it's really small, there's a true central square, and at night, everyone comes out there -- kids are running around, people are sitting on the low wall people-watching or chatting, while others are filling up the outdoor tables at the surrounding restaurants. The only other place I really saw this happening in Turkey during my summer of backpacking was in Ayvalik, where everyone gathered along the waterfront at night.
Once upon a time, Kas was the Lycian town of Antiphellos. There are a couple of bits left -- the theater is pretty well-preserved (local teenagers often hang out there at night) and there are some rock-cut tombs above the town. There are also a couple of sarcophagi scattered around town, the most well-known of which stands at the top of one of the shopping streets.
The town, by the way, is pronounced Ka-SH, because in Turkish, it has an accent on the s (but I don't usually use the Turkish characters on here -- too much of a pain on my American keyboard).