When DD was 16, I added her to a couple of our cards to help start building her credit. I was planning on doing the same for my son next year when he turns 16, but I ran across these offers on our AMEX cards. They are $50 statement if you sign up an AU and then spend $1K with 6 months on that card. Since I was going to do this next year anyway, I looked it up, and found you can add kids 13 and older, so I signed up my son for my BCP and DW's Cash Magnet.
It's basically a 5% cash back statement credit added to the existing rewards structure. I plan on running the spend through as gas on these cards instead of using my 4% PNC card. I'll basically get 6.5% on the Cash Magnet and 8% on the BCP up to $1K on each card. That will be easy to achieve in 6 months, and I really won't have to think about it.
It's too shabby for something I was going to do anyway on spend I was pay for anyway!