I'm not sure about the train route. The fast trains in Spain work best in a route that includes Barcelona, Madrid (branch to Toledo), Cordoba, Seville, and Granada. Generally, I'd suggest flying from Barcelona to Granada if you want to go directly. If you plan stops, renting a car is the next best option. If you don't want to drive, you may still be better off using the extensive, nice and cheap buses rather that using a train.
That said, here's my opinions on potential places to stop.
Velencia is a good choice for a big city.
IMO Belmonte Castle, somewhat inland but on the way if you go toward Valdepenas and Jaen out of Valencia, is the best castle along the route. If possible try to stop by there. Like most of Spain, they have siesta hours - you have to get there by a last admission time before siesta or wait a couple of hours to see the inside.
I really like Valdepenas, I think they produce the best Spanish red wines (my tastes lean toward Italian wines over French). Rick would describe it as a "workaday" town, very non-touristy. The only sight we visited was the wine museum. When we checked into our hotel, the desk clerk suggested it, then called a guy who opened it specifically for the two of us to visit. Spain is like that, especially outside the main tourist areas - they really go out of their way for visitors (which is why I now like Spain better than Italy).
We stopped in Jean but the traffic in town was really bad and the one sight we wanted to visit, the Arab baths, was closed for renovation.
Another good stop is Ubeda or Baeza. These are small towns surrounded by major olive groves. They were rich towns when New World money was flowing into Spain, then became backwater towns as trade routes changed - which makes them nicely preserved time-capsules from the time of Spanish conquistadors.