Following michael_p's excellent suggestion, perhaps check out IMMHO, some of the most enjoyable day walks in the ACT:
To Billy Billy Rocks from Smokers Gap ... a classic.
The Settlers Track ... simple but classy. There is good tank water at each of the tree huts.
Brayshaws Hut and Westerman #3 Hut on the Settlers track.
Lego Land ... something special for photographers.
Nursery Swamp, the largest fen on Mainland Oz. ... in a stunningly beautiful part of the ACT.
Boboyan Trig ... for the panorama-ists. Excellent sweeps.
Mount Gudgenby ... tells it all about what the ACT's ranges are like. Big Boro grounds up top.
Mount Namadgi ... if you have the legs? This is a monster of a day walk to another Boro Ground and back. A walk that you'll not ever forget. If you're fit, allow 12-13 hours solid walking.
Booroomba Rocks ... after reaching the top, at the saddle, don't think that that is all that there is (just a 30 minute walk), work your way west or further south-westerly to the higher points.
If you are mountain bikers, ride from Hall to Mulligans Flat on the Canberra Centenary Trail, and then return to Hall on the cycle path beside Lake Ginninderra. About 45 klicks all up. Definitely awarded 5 stars (by me). If you can do a car swap, it is a good walk between Hall and Mulligans Flat through the old Gungahlin State Forest. At Mulligans Flat, if time permits you can enter the animal enclosure and extend your ride through the enclosure to Goorooyarroo Nature Reserve, then head back along Horse Camp Drive towards Lake Ginninderra, then head back to Hall. This is now rated 10 stars ( by me).
A section through the old State Forest.
Sentry Box Hill on the Scabby Range ... to save time, ride your MTBs on the Grassy Creek Fire Trail past Westerman's and Waterhole Huts, before dumping your bikes in the bush near Luton's Crutching Sheds and then walk in a special part of the ACT.
Two shots from the Grassy Creek Fire Trail. Westermans #3 Hut and the Scabby Range in the distance.
There is a short walk called the Shanahan's Mountain loop. It is only about 3 klicks in total from the car park. It is very interesting and affords some of the best views in the region. You could do this walk in the morning and then walk the Settlers Track, that's close by, in the afternoon. Too easy.
Out near Googong Dam in the Nature Reserve there are some excellent low level crossings and fire trails to walk or ride to, like, to Gelignite Crossing and Flints Crossings over the spectacular London Bridge.
Gelignite Crossing.
Warren.