I second the Yoto. My son and I have had much fun making our own cards and I got pretty good at extracting audiobooks from YouTube, processing them with audacity and making cards of book series that he was into. You can fit a staggering amount onto a single card (5hrs of audio if memory serves).
Honestly that was the biggest extra feature for us, we quickly exhausted all the Yoto store content that appealed, and weren't into any of the big franchise content (except a pleasantly surprising read of Pixar's "Cars") or joining the Yoto club.
It's just an id. But the audio is stored on the yoto itself for offline play.
And second the blank/customizable cards, that's what 80% of our cards are and my daughter loves helping track down and extract content. Biggest hits for her have been Roald Dahl and random science stuff.
Roald Dahl has been great for the whole family, we do a lot of driving and listen to them in the car. We are very picky about the narrator's voice. Best so far have been The Witches (Miranda Richardson), Matilda (Kate Winslet) and the BFG (David Walliams - I don't like him personally but he is a great reader). Charlie and the Chocolate Factory was a disappointment, the narrator garbles and throws away the lines.
Also David Tennant reading the How To Train Your Dragon series. He is a superb reader with an amazing range. Before he hit the big time with Dr Who he did a lot of radio.
We've also had family members record stories and I've put them onto cards. We gave my son a dictaphone to record his own stuff. He would do great long sagas of mayhem and battle, but he's lost interest in recent months.
Honestly that was the biggest extra feature for us, we quickly exhausted all the Yoto store content that appealed, and weren't into any of the big franchise content (except a pleasantly surprising read of Pixar's "Cars") or joining the Yoto club.