Mike,
agree with the other suggestions but would add the ferry over to south perth (next to the Bell Tower*). View back to the city, especially at night, is nice and there are some nice pubs in south perth.
Other thought is that it is the first Tuesday of the month so there will be an AGWA meeting. PM me if you would like arrange a lift.
Dave
*best description I have heard of the Bell Tower is a 'cockroach doing something to a praying mantiss'
. More seriously, the display at the bottom has a very interesting transit scope.