Thanks all for your comments. Given the current financial mess and not enough time, I think we'll skip Darwin this time and put it on the wish list for a longer land tour of Australia, when diving is not the main event.

Looks like we'll keep three days open to explore north of Cairns before meeting up with the dive group and look for bargains a little closer to April. Since DH is always saddled with driving and does not want to do it this time, we'll check out various guided tours which you've given us links too, as well as some I've run across in my research.