Originally Posted by Auscal

Sydney to Nowra to Bowral & back to Sydney is way longer than 2.5 hours. IMHO, it's more like (& these times are approximate), Sydney to Wollongong is 1 hr, Wollongong to Nowra, almost an hour, Nowra to Bowral (going through Kangaroo Valley) is about an hour - and it's two winding country passes, so, not a relaxing drive - then Bowral back to Sydney is another hour.

I could see doing Sydney to Wollongong (Royal National Park, Grand Pacific Drive), then to Albion Park (southern most suburb of Wollongong), up Macquarie pass (8 winding kilometers) through Robertson (where the movie Babe was filmed), along the Illawarra Highway, then turning to go through Bowral (upscale yuppie country town where wealthy Sydneysiders have their weekend homes) and getting back on the Hume Highway to Sydney. That's probably about 2.5 to 3 hours driving.
If you wanted to make it a bit longer, and more "country" you could keep going south on the Princess Hwy past Wollongong to Kiama, (and see the only blowhole in the Southern Hemisphere) which is a pretty seaside village with a nice harbour - then possibly visiting Minnamurra Rainforest - if you'd like to walk through the rainforest, then travel via Jamberoo (small rural country village) up to Robertson (visiting the Illawarra Flywalk on the way - a walk high up in the rainforest), and connect with the Illawarra Hwy there (near the top of Macquarie Pass) and carry on to Bowral and Sydney.
If you pass through Robertson the Village Bakery is a great little hole in the wall bakery, and Pizza in the Mist offers really good wood-fired pizza's (but, possible only in the evening, my memory fails me). There is also Robbo pub (the Robertson Hotel) And, just a few miles past Robertson on the Illawarra Hwy, you'll see a turn-off to Burrawang, it's only a couple of kilometres detour, and a great little country pub.

To the OP, have a look at a map, check out some websites, and see what appeals. North of Wollongong, at Helensburgh, there is Symbio Animal Park, if you need a dose of kangaroos & koalas, in a smaller more relaxed setting than some other parks.

Heading south from Sydney, then to the Southern Highlands is a full day, but, you can see a variety of scenery - coastal, rural, highland, and more generic "australian bush". It is a full day - you should stop & get out the car, not just drive. Trying to include the Blue Mountains would not make a lot of sense, and would be an exhausting day spent at the steering wheel.

I grew up in Wollongong, then moved to Robertson, so know the area well.