Just about anything, especially if the shy and quiet part is a facade.
But let's say it's genuine for the sake of this post.
I'd say lots of masturbation, voyeurism, and maybe some exhibition.
1. Girl is going a class trip and seems oddly excited about it despite being stuck in a crappy bus with old shocks. Her friend, who is sitting next to her, notices her getting increasingly flustered as the trip goes on. Friend confronts her at a stop and girl reveals she has a butt plug inside her and all of the vibrations go straight to the base and then inside of her). Friend is turned on by the idea and teases and fondles her the rest of the way back home, making her come over and over while no one is the wiser.
2. Shy girl masturbates in public places, the riskier the better.
3. Shy girl likes masturbating to camgirls. She tries a new site, starts fingering herself and is shocked when she find something out the site is two way: they can see you once you go into a VIP room. Performer flips the script, directing her through a mutual masturbation session and a little submission.
4. Shy girl likes wearing nothing or very skimpy lingerie under her clothes and then "flashing" imaginary passerby (she exposes herself but no one is around to witness it). One time she does so and finds out someone is watching (maybe they were behind tinted windows, maybe their car wasn't in the driveway, etc)