I would count Nursery swamp out. great day walk but you wouldn't want sleep around there at night. too many mozzies.
Bushold flats is a nice spot and an easy walk from the base of Mt Tennent.
I can also highly recommend walking along the Rendezvous creek firetrail until it ends at about 6kms. its a lovely, flat walk with lots of options to set up camp. lots of water as well.
The walk to Horse Gully hut is also very nice, although a little longer than what you wanted, about 9km each way from memory. lots of water and easy to walk. beautiful camping in the Naas Valley.
What time of year were you planning on going? Summers in Namadgi can be oppressive with the heat and the flies.