G'day Alan, considering you are supposed to be at the airport '-2h' before international flights, it's not worth leaving the airport, with all of the runaround of getting the ETA and having to be back within the hour. Any delays will kill any chance of an activity in that time.

If you're able to access the Koru Club (just a guess, unless UA can block it) you will really enjoy your time there. There's nothing interesting nearby unless you get a taxi to Brighton le Sands, which would be peak hour, so fuggedaboudit!

If you could swing it, the QF F lounge would be the choice option at SYD.