We chose both the Retiro garden and the Royal Palace option. There were only four tourists in the garden but 24ish in the palace. It was a long day, but having a guide helped put the history into perspective. He was knowledgeable and humorous.
Excellent work, Alex.
A highlight was seeing the Stradivarius violins and musical instruments. Photos not allowed in those areas, only in the entrance halls and stairways.