Three times a year I help instruct on the HEAT (Hostile Environment Awareness Training) course run by the German Government. At the end of the last one,Scott a United Nations official gave a speech to the participants he talked at one point about the necessarty of being fit.
He said that if you were not fit on Mission that you could be a danger to yourself and your colleagues, perhaps with fatal results. At home there are lots of ways, but on Mission? Maybe it would break the boredom but what if there is nothing to help or, it´s too dangerous to go outside? Well I remember in the Military on Mission we had a multi gym and a Sauna.
It was a first time for me but one lesson I will never forget is don´t wear Dog Tags (or anything) on your neck in the Sauna or you will regret it!!
I found something which is small and light with a big effect. It is called a Sling Trainer sometimes Suspension Trainer or the Original was the TRX.


The TRX was invented by a SEAL (Naval Special Forces USA) who by accident took a Belt from his Martial Arts uniform/Gi with him on Mission when he discovered it in his bag he was at the “Hurry up to wait” stage and had time on his hands, so he worked out excersies with it and so the idea was born.
Now if you look on the Internet you will see a lot of prices up to around €180! if you give in Suspension or Sling Trainer you might be around €50,
Still quiet expensive, well you can get them cheaper sometimes at Discounters, in Germany Aldi or Lidl, I saw them for about €19 and then reduced for €8! Thats cheap, I was Lucky. Really they are just like Nylon belts which you can put over a door which closes TO YOU, There is a warning sign for people not to open the door while you are Training.
Also you can unclip the part for the Door and use the carrabiner to afix it to a Tree. With the Sling you can do quiet alot of excersises. Here are some Screenshots from Youtube Videos

For those who enjoy making things, here is an even cheaper way from Budser at Instructable.com Enjoy