Shame you don't get to Melbourne, so you could ride Beach Road. Next time.

Sydney - certainly the Opera House, and I'd suggest a Harbour cruise and maybe dinner over at Darling Harbour, but I wouldn't count on a lot of time at The Rocks unless you like tourist shops. Sydney has a red tourist bus, called Sydney Explorer I think, which is really good (and includes the Opera House). You get a ticket, and hop on and off at nearly a dozen locations during the day. It just keeps doing circuits, so you can get your bearings, without the hassle of getting to everything separately. Also, try to get to Bondi, Australia's most famous beach, which is just a few km east of the CBD. The Blue Mountains are beautiful.

Tassie - the wilderness areas are Tassie's jewel, and if you're into hiking, there'd be nowhere better. I'd do that over Port Arthur personally. Maybe Cradle Mountain too - there's a good web site, some fabulous hikes and food. Lake St Clair is a good idea too. Tassie is very pretty, and small, so you can get around pretty quick.

Have fun.