My players did similar. At the end of Act 3 they gave me a mini of the dragon they killed (we used it for the battle where it died) and they are doing the same for the Act 5 finale with two dragons.I painted a miniature of Strahd for the DM who ran my wife through her first campaign - Curse of Strahd.
I'd like to do the same, but I'm not going to be able to do the book - I'm thinking of a t-shirt with some of the campaign art from the APs/modules. It is an almost 10-year campaign.I usually give them a token of my appreciation at the end of a long campaign. Last time I gave them a book with every adventure we went through.