With a lot of time and little space, they made miniatures.
Below is a video of German prisoners of war in the US in 1944 making a complete model town…for turtles. Shirtless and smoking, they take turns adjusting miniature items and prodding the animals. Dare I say, looks like fun?
Meanwhile in Mississippi around the same time, German prisoners helped make the Mississippi River Basin Model, the largest small-scale model ever built. The finished product allowed the Army Corps of Engineers to experiment with different levee systems. It involved a lot of labor, for which the prisoners were given 90 cents for every 8 hours.

But these Germans aren’t the only POWs in history to take on miniature projects. WWI had its fair share of crafts as well. This figurine, for example, was carved by a prisoner at a camp in Scotland in 1919:

As far as artistry goes, though, the Napoleonic Wars (1793-1815) outshine the rest. French and other prisoners living in camps in England sculpted models, dollhouse miniatures, and other objects, mostly from bone they collected from their food rations. Cash earned from the sale of miniatures helped them battle poor living conditions. Here’s one prisoner-crafted, working replica of a French guillotine:

The prisoners also made dollhouse accessories, like this tiny dominos set:

And they capitalized on high demand for dollhouse furniture depicting lavish interiors:

Crafting miniatures allowed POWs an escape, a refuge from an otherwise dreary camp life. It was a natural choice of artistry under their circumstances, as it required few supplies and little space. And it gave many of them a chance to earn pocket money as they awaited repatriation.
Making a model town for turtles, though? That may have been just for fun.
Via Atlas Obscura and POW Crafts