Don’t charge more than a 1/4 of the credit limit if possible.
I did authorize my D on one of my credit card before she got her Discover card. Her Discover does earn cash back and has the double cash back offer on the first anniversary too, it just does not have the quarterly 5% offer. It does have those Discover Deals offer you though. So she may get 10% cash back at Samsung or Apple.com purchased.
When I mentioned a car, I was thinking along the lines of a gas credit car- -a Shell or Mobil or whatever card.