My family is from a town near A Coruña. I was there this summer for a visit. The public transportation system is great in the city, We used the buses a lot and we also walked all around the city.
La Torre de Hercules is a UNESCO World Heritage site and definitely worth a visit. To get to Santiago you can take a bus from the bus station located in front of the Corte Inglés dept. store. It is very convenient!
A Coruña has a wonderful promenade on the water that leads you to the wonderful Aquarium. The María Pita plaza is also beautiful! There are restaurants and stores all around. I don't have a recommendation for hostals, unfortunately. However, I am certain you will find a good place to stay. It is a city that is ready for tourists.
Enjoy Galicia!! I hope you like seafood! If so, you will enjoy Galicia for sure!!