Any suggestions for a weekend in or near Sydney for my college age son and myself in mid-October. We can travel and have previously done a lot of the major sites. Thinking of something off the beaten track-any ideas that he might not do otherwise.

Houseboat/cruiser/yacht charter on pittwater or the Hawkesbury.

drb - I have 3 suggestions, firstly
* Hang the expense, fly over to Lord Howe Island, and make it 3 days or even a week.
* Drive down through the Southern Highlands, take the road to Roberston, then to Wildes Meadow, then Burrawang, then Fitzroy Falls, onto Kangaroo Valley (lunch at the Friendly Inn), then across Camberwarra Mtn, to Jamberoo, then to Gerora or Gerrigong (overnight), then back to Sydney.
* Why not a drive out Mudgee, take in a bit of history and wine.

If you have a car, try driving down to Mittagong then taking the road across to Wombeyan Caves (an experience in itself), then on to Taralga, Crookwell and up to Abercrombie Caves.
Both sets of caves have easy self-guided tours or guided tours, and both have accommodation.
Then up to Bathurst - not forgetting a circuit of Mt Panorama, and back across to Sydney.

The drive and the scenery are as good as the locations, and as long as your not in School holidays you'll have the places to yourself.

I forgot to mention that even though a few stretches of road between Mittagong and Bathurst are unsealed, they are still well graded and easily done in a 2 wheel drive car - and if you have an extra day it's well worth going on to Hill-End and Mudgee

I'm not sure if you are interested in wildlife, but I did this trip in an ordinary car two weekends ago and saw about 5 different mobs of kangaroos , a couple of wedge tailed eagles and an emu near Bathurst !