From a born and bred Canberran who visits regularly I would also consider adding to your list the following;
Pochers Pantry, it's a winery and smokehouse on the road out to Yass. The food is exceptional and the wine is pretty good too.
A day trip or overnight to the Snowy Mountains is a must. You could consider extending the trip to a 2 day event do the walk to Mt Kosciuzko from Charlottes Pass and the next day follow the Alpine Way through Thredbo and on to Albury. The drive down from Thredbo to Albury is breathtaking. Tons of photo opportunities of not only nature but also the Snowy Mtns Hydro Electric network of tunnels, dams and power stations are well worth it.
Telestra Tower on Black Mountain.
If you are really hungry for a good home cooked meal visit the Central Cafe in nearby Queanbeyan and go a mixed grill, I dare ya
.
While in Queanbeyan you could visit Molonglo Gorge, off Sutton Road on the way back to Canberra via the Airport Road. A drive out to Bungendore while in Queanbeyan, follow the Coast Road (Kings Highway which is the continuation of the main street in Queanbeya going North) for about 30 minutes to this beautiful village. Magellan Observatory is at Lake Bathurst another 30 minutes North on the Goulburn Road.
On the topic of food, Manuka and Kingston will satisfy your appetite with varying cuisines in restaurants and cafes.
A drive out to Lake George, on the Federal Highway to Sydney, for some photos followed by wine tasting and lunch at the wineries out there make for a good day too.
Plus all the other places others have posted. The War Memorial is a must particularly so close to Anzac Day.
As far as your accommodation goes, I would check parliament sitting days around then and try to get your accommodation away from those days. Varying your travel plans by a day or two could save you a pretty penny, not only in accommodation but car hire as well - its the old supply versus demand pricing model in both industries.
A biased opinion but it really is a beautiful place. Maybe a bit sterile for people who enjoy the hustle and bustle of Sydney or Melbourne.
Wayne
PS - don't forget to pack your GPS if you have one. If you don't have one get one with your rental car. Canberra's road network is first class but it can get confusing with all the roundabouts, particularly if you are not used to them.
What else to do in the ACT when visiting NACAA 2010?
(including a bunch of stuff like accom, car hire, flights, etc), quite a cost which could be saved for future astro gear 
so consiering. Think I will get swayed in to going over anyhow as my wife wants more holidays.. but ouch, bit of a cost. Hmm.
