Search the forum for things like "4 Motion" and "suspension". You'll find tons of discussion about all aspects of off-road etc.
But in brief: in its base form a Cali is inherently little better or worse in terms of off-road mobility than any other panel van type campervan.
As with any vehicle, tyres are always the first upgrade for off-road use, but then there is also the seasonal dimension.
You can opt for 4WD/AWD (4-Motion - the VW factory option). As aftermarket options, depending on your budget, you could look at raised suspension kit (Seikel) or air suspension (VB). Those will all increase the vehicle's capabilities off-road but in the end it's a commercial van, with limited suspension travel and approach/depart angles.
And of course it's always a trade-off: 4-Motion will have a permanent negative impact on fuel economy, suspension lifting will have a negative effect on road handling (higher centre of gravity) and air suspension will cost you something approaching five grand.
What type of off-road use did you have in mind? Muddy campsites or serious over-landing?