How can I encourage her to take off her hat?

My partner's 14 yr old sis, Leo, is quite tomboyish, sporty and athletic. she's very into football and all kinds of sports like that. i get on really well with her, go to watch her play for her under 15s team on a sat and sun morn and we have quite a laugh together, usually at my expense, but there we go, lol! Anyway, she's got this gorgeous, long, strawberry-blonde hair that is just scrunched back into a quick ponytail for footie or covered with a baseball cap any other time. she's hadly ever out of this cap, which is old, faded and pretty gross - i swear she sleeps in it too! I don't want to ask her to take it off cos thatd feel lyk im undermining my partner, who incidentally is always in a cap tpp, but id love to see her with just her hair down, rather than hidden away. being under a cap all the tym cant be good for her hair either? any sensible suggestions anybody please?