Kids snail mail business advice

Hi everyone. I make a children's snail-mail subscription where kids receive handwritten letters, illustrations and other surprises through the post each month.

I'm trying to figure out what would make something like this genuinely exciting for a child and worth paying for as a parent.

So, if you were going to subscribe your child to a monthly letter club, what would you actually want them to receive?

Would it be:

• A continuing story?
• Activities or puzzles?
• Stickers?
• Collectible characters?
• Something they can make?
• Something completely different?

What would make you think, “Yep, my kid would absolutely love this”?

Thanks so much for your time