Canberra is so full of attractions that it would take several weeks to see them all. What that means is that you should have no problem enjoyably filling the limited time allocated to sightseeing.
To gain an overview of Walter Burley Griffin's planned city, head to the top of Mt Ainslie, from where one finds a perfect lookout over the north-south land axis around which Canberra is based. If you can, take the time to stroll along Lake Burley Griffin, the artificial waterway that cuts perpendicularly across the land axis, and which is right near the CBD. Then there's Black Mountain Tower, from where one can gain a broad panorama of the city.