I think
Harriet, especially with nickname
Hattie, is DARLING. It's old-fashioned and cool, and I like that it doesn't end in -ia, -la, -ana, etc.
Harriet Amy is very nice, but I'd use something a little spicier in the middle.
Harriet Lilith,
Harriet Cordelia,
Harriet Astraea,
Harriet Mehitabel, etc. Okay, maaaybe not the last one. But you get my meaning.