One of the most popular World War I collectible is Trench Art. The term encompasses objects made from the debris and by-products of the war. Decorated brass shell cases, being available by the millions on the Western Front, are the most recognizable.
Above are two images of a German 77mm shell casing turned into a memento of the battle for Montfaucon. One side has been inscribed with "Montfaucon 1918" and the Corps of Engineers castle insignia in relief on an embossed background. The reverse is decorated with "304th E." [Engineer Regiment] and the badge of its parent formation, the 79th Division (a Cross of Lorraine within a shield) on an embossed background.
This specimen is from the collection of Trench Art expert and author Jane Kimball. She has published an outstanding source book on her specialty, Trench Art: An Illustrated History, which can be ordered at Amazon, Atlas Books, or at her excellent website: www.trenchart.org
Here is an amazing specimen — a functional clock — I saw firsthand at the the Hill 62 Museum at Ypres on my recently battlefield visit. (Photo from Jane's site.)