http://tp.transportnsw.info/nsw/XSLT_TRIP_REQUEST2?language=en
this site is useful to help you plan trips on busses, trains and ferry's etc.
As others have indicated the two main venues close to the ground are the Brookvale hotel and the Manly Leagues Club.
The leagues club does have some memorabilia on show, does reasonable food and drink prices are OK - there is also a Sea Eagles merchandise retail outlet across the road upstairs from the car park (ask door staff at the club for directions) which would be a good place to get gear and pressies for home.
One option as mentioned would be to come over early by ferry from circular Quay (use the tool above for time to Manly Wharf) and spend some time in Manly.
You could easily make a day of as there's plenty to do
http://www.tripadvisor.com.au/Tourism-g552103-Manly_Greater_Sydney_New_South_Wales-Vacations.html
The corso (which is the main pedestrian thoroughfare between Manly Wharf and the main surf beach) has plenty of food and beverage options for a leisurely lunch.
There are a few good "Chippies" on the main beachfront if you'd want to watch the surf and have a feed by the beach.
Turn right at the end of the corso and follow the path along shoreline and you'll get to Shelly Beach which is a nice walk.
Or, enter the pub (The Steyne) on your left at the end of the Corso and have a pint and watch the world go by (Ask staff how to get up to the cider bar which is upstairs and you can sit on the balcony and get a great view of the beach and surf.)
You can then get a bus from Manly Wharf up to the Manly Warringah leagues club (bus trip is about 20 minutes and will cost around $3.50)
Post game the Leagues Club is good for a beer - especially if we've had a win!
Safe travels and enjoy the trip.