what about sending them a gift basket that arrives the same day they get home? I know Hallmark has gift baskets, and you can include a card with it. Not sure about getting it to Australia, but surely in this day and age that's not a big deal.
I think it would be nice to arrive home to a nice basket and thoughtful sentiment.
If you want to actually send them home with something, my suggestion is you print out a picture from their time here and put it in a frame that says "vacation" or "family" or something of that nature - so you are sending them home with a very real reminder of their trip and a great pic of everyone.