All lovely names. I wouldn't dismiss Maria (unless you actually live in one of the countries where it's so popular?) It's quite rare in children here? I've taught several but every single one has been an international boarder (Spanish). It does seem like, if that limited experience is anything to go on (which I'm sure it can't be), about 20% of Spanish 12-25 year olds are called Maria!
When I think of youthful, informal and not twee, I think:
Astrid, Anna, Aria, Ava, Aubrey, Avery, Bronte, Bella, Bethan, Carys, Cleo, Cassidy, Delphi, Dana, Effie, Etta, Eden, Everly, Faith, Fenella, Giselle, Hallie, Hope, Isla, Ivy, Juniper, Jessica, Kiki, Kyra, Livia, Lara, Leonie, Mika, Mala, Miranda, Nova, Nyah, Piper, Pippa, Quinn, Demi, Romilly, Ramona, Robyn, Saskia, Seren, Saffron, Thea, Tamara, Verity, Wren, Xanthe, Zara, Zeryn, Zia