Santiago is a jewel! It is a lovely medieval town with a lot of architecture in stone. The crown jewel is the Cathedral. Just meandering around town is so great. I have been there twice for 4 days each time . One day we took a tour to Costa do morte ( Finisterre) and another day I went to Coruña. Another possible day trip could be to Pontevedra.
Both times I have been there in July. The first time in 2002 it was cool in the evenings and early morning and it warmed up to the the 70's by noon. Last year There was a heta wave in most of Northern Spain and temperature was in the high 80's! I was very happy to have A/C in my hotel!