As both Cervantes and Shakespeare were extremely prolific writers, who addressed diverse themes, it is somewhat difficult to summarize all of their works in a simple fashion.
Both writers dealt at length with love. It is a subject that appeared in the plots and subplots of most of their works. In general, their treatments reflect the predominant Christian views of the time, in which adultery was unacceptable and normally punished, divorce extremely rare, and a "happy ending" to a love plot was a marriage. Cervantes satirizes the earlier mode of chivalric romance that existed outside marriage.
ANother major topic in both writers was the operation of power and its conflict with morality, especially in the way characters might become corrupted by ambition and the search for power.
A third theme addressed by both writers is aging and mortality, and how humans ought to respond in face of them.