I like
Cassie better than a lot of diminutive names like
Sophie,
Carrie,
Millie,
Bella,
Marnie,
Mila, and
Elsie. It is very dated to the 80s and 90s though. On the plus side, I don't associate the name with just one individual since there are several minor tv/book/comic characters with the name, but none that really standout to me over the others.