Namadgi National Park has not been mentioned much yet on canberra.bike. After the fires, it has remained closed. The northern and southern tips of the park are now opening again.
Tharwa is a great idea for a day out and there is a lot to see in a small area. You could consider riding there south through Canberra, or if you are keen then try the loop ride from Stromlo Forest Park south through the suburbs to Tharwa, where you cross the Murrumbidgee River and return to Stromlo Forest Park on the other side of the river via Tidbinbilla Nature Reserve and Cotter.