There must be hundred of things you could do. Here is a list of a hundred from Innocent Drinks:
I remember a SERE lecture in which the RM Sgt instructing us commented that keeping your mind occupied is important. You can find similar open source studies of Vietnam POWs that also say this. One guy mentally planned, built, and started to run a hotel.
Since you have aces to pen and paper, and PC/laptop/phone - why not think up a new idea for something like a mail order business selling things like Larne Sausage or Scottish Black Bun? Or Shetland Loaf?
During quiet night shifts on a tour many years ago I got bored and designed a street legal dalek (based on a very cut down mini chassis) a bike and side-car with the controls in the sidecar, then refined it to include a BMW flat-twin engine in the back of the sidecar.
After that I got silly