Things to do.

Mar di Cristal itself is small.  You can walk around easily.  The Puerto has nice bar & tapas and if its not too hot, you can walk to neighbouring villages.  The beachside properties in Los Nietos are quaint, and in the opposite direction to La Manga camping  you can follow the seaside path right round to Cabos and the beginning of the La Manga strip.   The strip itself is full of bars, restaurants and beaches. 

Cabos de Palos is a pretty harbour.  Lovely restaurants and a gentle walk around the peninsula to the lighthouse.  Good views of the La Manga strip

Cartegena city is beautiful.  Easy to drive or leave the car at Los Nietos station and get the little train which takes you right into the centre.  Follow the city walls and walk down to the port.  From there, you can stroll into the city itself.   The Civil war museum, Castle and Roman Ampitheatre and museum are close by.  Ask for deals on the tickets and you can gain entry to more for less.  If you're happy seeing the amphitheater/castle from the outside, the castle gardens don't require a ticket and have great views over the city. 

Further up the coast, Los Alcazares and Los Narejos are busier places   The hotel L'Encarnacion is a rare old building with a beautiful courtyard and baths, serves wonderful food and has real flamenco on Sunday evenings. Booking essential

There's natural park and mud baths at San Pedro del Pinatar, although I'm not sure the weather will be warm enough in February.  Flamingoes frequent it in warmer months. 

Trip Advisor would be a good place to start browsing what may interest you   

Thank you, looking forward to my trip even more now!!.