When the party has died down, Isabella, for one, is well and truly exhausted. She explores the palace until she finds a room with a bed in it, and into this bed she flops, still in her clothes and holding her staff and carrying the cordial in her pocket. She sleeps late, because the party kept her up so late and she hadn't really slept the night before; but around noon, she stirs, and gets up, and goes looking for James and wherever her backpack may have got to. The backpack she finds in the great hall where the principal mass of the party was; some enterprising creature took both bags from the battlefield at Beruna up to the castle for them, and she only wishes she knew who it was. She takes her bag to her room and carries James's with her and continues looking for her friend.
And so midnight finds them perched on their thrones with tea and a plate of cookies for Father Christmas if he should happen to want them.
"And James, Eve's Son. The wells damaged in the long winter have been repaired, and here is a little something to keep your mind occupied." He hands her the last object he was carrying - it's some kind of decorative puzzle, currently in the form of a tetrahedron with each face made of four adjacent triangular panels. The panels are made of various metals - gold, silver, copper, bronze, brass - and engraved with intricate geometric patterns.
"Thanks!" says James, resisting with considerable effort the urge to start playing with it immediately.
"Do you always come at midnight?" Isabella asks. "And how do I go about finding particular pages in my infinity notebook?"
"Thank you!" Isabella calls again, beaming and hugging her notebook.
"I wonder what he meant by 'loyal companion'," muses James. "I got the sense it meant a thing. Maybe it follows you around, or can only be read with your permission, or something."
James takes the book and tries it. It opens just fine. She passes it back.
"Huh. I should've asked. Maybe it just means it's durable and I'll still have it in years and years."
"I don't think I will do that, no," agrees Bella. "What the heck is your thing, besides pretty?"
"Okay, that's cool," says James. "And I'd say it was way too simple, but there are more colours than sides, so I'm not even sure what would count as a solution. I'll play with it, I guess."
"Have fun," laughs Bella. "I'm gonna go to bed and in the morning I'm going to copy all my important long-term notes into my infinity notebook." She hugs it. It's white leather-bound with gold corners and an embossed gold outline of her scepter and crown on the cover, and only about an inch thick despite its reported page content. Off she traipses. "Merry Christmas!"
Isabella has a bag made - the backpack doesn't keep things near enough to hand for her liking, and anyway it looks very odd against the sorts of outfits the dryad wardrobe mistress likes to put her in (whether it's breeches and boots for riding or fine queenly garb for formal occasions). The bag, painted silk (for its strength and softness both) slung messenger-style over her shoulder, has a loop to tuck her scepter into and plenty of room for infinity-notebook, pen, and cordial, as well as any incidentals she picks up.
In the spring (which is met with ecstatic relief by the populace) when it is again possible to build things, the rulers of Narnia establish a handful of schools, although unlike those they remember from Earth these neither favor specific ages (which would be quite absurd in as diverse a country as Narnia anyway) nor demand attendance from anyone who does not care to go. These are soon turned into little library-study-halls where people who know things and people who wish to know things congregate. Isabella allows a dwarf to copy bits of her notes to compile into useful books; it turns out that the infinity notebook will cooperatively open to whatever page she is looking for, but it will do this only for her, and so she doesn't much mind having it in the hands of someone else as long as she opens it to begin with to whatever she wishes to show them.
James's birthday, calculated by subjective days, turns out to fall on Narnia's May 7, and when this is announced it is summarily fused with the nearest preexisting national holiday, a celebration of consistent warmth and sunshine and full of all the usual feasting and dancing and games associated with holidays generally. It is also customary to leave anonymous baskets of small presents ranging from flowers to practical gifts on the doorsteps of neighbors on the previously existing holiday. It is renamed "Kingsday".
Isabella doesn't recalculate her birthday, even though it would have fallen at a similar date; she just "turns twelve" four months after James turns thirteen, under the changing leaves. This allows her to co-opt a harvest-and-arts-and-eating-desserts festival associated with large impromptu markets for trade and display of various crafts and other things ranging from preserves to maple syrup, which suits her just fine. She eats desserts and browses art with the best of them. It is, to match James's, called "Queensday".
The giants to the north make a little trouble not long after this holiday, sensing a soft and juicy target, but the Narnians aren't as soft as all that. A small minority of giants are willing to meet with James for diplomacy when a letter is dropped into their camp by an eagle messenger, but the rest of them have to be driven away by force; James's sword and Isabella's cordial come in handy once again, as does Isabella's unicorn bodyguard from the Battle of Beruna. Eventually the giants are routed from the borders of Narnia.
And eventually Christmas comes again.
Isabella stays up.
When midnight rolls around, she is busy trying to find another solution to the cube shape.
"Merry Christmas!"
"Isabella, Daughter of Eve. You have been making good use of the fire-berry cordial, but its supply is not infinite. Keep this plant where it may catch the light of the rising sun, and when you see the level in the bottle begin to drop, pick one of the berries in the morning while it is still shining and put it into the bottle. They are not true fire-berries, but they will serve this one purpose very well."
Next he withdraws a smallish rectangle of card or paper, almost like a postcard. It seems like ordinary paper, if of a very high quality, all around its edges - but in the middle it is transparent, showing the texture of the paper only very faintly over a perfect view of whatever is behind it. He hands it to her along with the potted plant.
"This page will capture a picture if you look through it and wish it so. If you set it on top of ordinary paper, or any other surface that can be drawn or painted on, and wish it to copy its picture there, it will; if you hold it and wish it to clear itself so it can be used again, it will do that."