I adore so many of the suggestions so far!
A few more ideas:
[name]Peter[/name] - sweet on a little boy, but ages brilliantly, and a classic, but never boring. I find that it holds all of the same qualities as [name]Thomas[/name] (and is almost 150 places lower on the SSA list).
[name]Harriet[/name] - perennially popular here in [name]England[/name], and for good reason, but doesn’t even make an appearance on the SSA’s top 1000 in [name]America[/name]. Such a classic, certainly feminine but not at all frilly, and just look at the host of nicknames, which would suit all sorts of personalities and all ages - [name]Harri[/name], [name]Hattie[/name], [name]Happy[/name], [name]Etta[/name], [name]Ettie[/name].
[name]Mira[/name] - this sort of short, simple and sweet name does a very good job of being feminine without overdoing the frills, and also often seems to defy the ebs and flows of popularity which dates other names. [name]Nora[/name] has a similar feel.
[name]Sarah[/name] - such a classic. I know Sarahs of all ages, and it suits everyone. Plus, [name]Sadie[/name] is spunky and [name]Sally[/name] is vintage sweetness as far as nicknames go. I also love [name]Sara[/name] (SAH-ra, as opposed to SARE-a), but it is very open to mispronunciation.
[name]Julia[/name] has been mentioned, but I must remind you of the equally lovely alternative [name]Juliet[/name], and also the male equivalent [name]Julian[/name]. And if nicknames are important to you, the lovely [name]James[/name] can be not-particularly-shortened to [name]Jem[/name], which I think is very cool 
Good luck!
[name]Auburn[/name]