Well, I plan on getting him something for herblore. He has 84 herb.

Give him some coconut milk, to make pots :

Good Idea, thanks. I might also give him some crushed bird nests.

Just five him a few thousand vials. They are always uselful for putting the potions in.

Yep you could give some vials too or a santa hat

Vials are are always helpful, Could give him unids or other herbs you don't want as well.

I find money to be a good choice. it turns into any present they want!

Giving runes or vials is like giving an irl friend soap. Sure they might want/need it but it just doesn't make a good gift. Find how much you want to spend, and buy something flashy. Tell your friend he can sell it if he needs money.