Skateboarding

Believe me I know full well that feeling of not wanting anything to happen when you live in your vehicle and all your possessions are contained within. For me I was more worried about the vehicle itself braking than my body. Whilst I have mechanical knowledge and did carry some tools around with me my first van the engine blew and that was a less than fun situation being stranded and having to essentially leave your home with someone to fix and thus not being able to use your home. It was weird.

You said you are going to be Australia though, on the east coast most places have decently paved stretches along the beaches and the like so use them to get used to just cruising on your board if you have no prior skating experience. There you can get used to the way the board feels and just get comfortable. I did the same, living out of a van on the east coast and I used to cruise all the time as it is a beautiful places to do so and like you say a skateboard doesn't take up much real estate in a van (unlike my two bikes which was a decision I regret at points xD)

After you are more comfortable you can graduate to trying out more parks or just hit some bowls and learn to carve them a bit. As long as you slowly ramp up with the levels of risk and danger I think you will be alright and you won't regret getting out there and getting involved.

More than anything have fun :D (I'm fucking jealous xD I miss my van life even with the uncertainty it sometimes bought)