Besides getting myself into good shape. I would get a whole bunch of certifications that are related to your industry. HN doesn't like certifications, but boy, defense industry loves that stuff.
I second this, because you will have a goal to work towards and something measurable afterwards. If you are not really inspired, many other suggestions might be harder to really follow through on, so you might end up with having done nothing in the end.
By all means, do relax, but I‘ve often found it hard to do so, when you are sort of required to do something but actually just waiting. I have often found those situations to be more taxing than actually working.
I'm out of defense for more than 2 decades now. Back then, MSCE, Java and Oracle were the rage. Just look up some job descriptions. They love the soft stuff like scrum master, agile, etc.. They need that check boxes for contracts.
By all means, do relax, but I‘ve often found it hard to do so, when you are sort of required to do something but actually just waiting. I have often found those situations to be more taxing than actually working.