Thinking along the lines of getting a car and depending on where you live:
1. Car wash vouchers. It's really nice to be able to wash one's car on a whim.
2. Emergency car kit. Decorate a cute bin and put useful stuff in it: first aid kit, simple tool kit. blanket, paper towels, etc. It's not the most exciting thing; however, it's practical, and her mom will love you for it.
As for travel gifts for a 16-year-old girl and depending on where they like to travel:
1. Beach tote with a cute beach towel, sun block, flip flops, and hair ties
2. Hip bag or crossbody. Put small things like chapstick, hand sanitizer, mini wallet, little fun things, and even a little bit of cash if you want.
3. Cell phone accessories
That's all I've got. Good luck coming up with a gift idea. For me, making a gift with a theme and choosing little things that go along with the theme seems to work best. I learned it from my mother-in-law, and over the years, so many people have loved her gifts.