I'm personally fond of bodyweight exercises. You are your own gym and convict conditioning are both really good resources. Also look up Al Kavadlo – We're Working Out! | We're Working Out! for video tutorials and articles. I personally would think twice about Crossfit if you are only going to be able to go once a week. I would be concerned that the risk of injury is to high. Part of injury prevention would be getting your body used to the high level of work, and I'm not sure once a week is enough. But ultimately only you know what your body can handle.