Correct me if I am wrong, but doesn't Cassandra enjoy romance stories? In fact, with a male Hawke, she enjoyed the part where he fights the Arishok, especially one on one, and seems to have briefly developed a crush. Even with the female Hawke, she develops a bit of hero worship.
She is tough, no question about it, but I think Cassandra will show a softer side. Knowing she likes romance stories may suggest that she will be reading some books like that in her downtime. I don't know what Vivienne likes to do outside of fighting except show her love for fashion, so I can't say my thoughts on that. The point is, with Cassandra, she does seem to have a relaxed downtime. If the saying about loving romance stories means anything to her hobbies in Skyhold.
I am female, and I love the characters in the games, both male and female. They each have different characterizations--some I agree, some I don't--even though DA2 doesn't mention much on relaxed hobbies. In Origins, you do know with Leliana that she loves to tell stories and write songs for different adventures. Morrigan may not seem to have much hobbies, but she is also raised by Flemeth who has her do questionable past times. But, she does seem to like to shapeshift to her animal forms, so maybe you can say her hobby is walking through forest areas as animals?