It used to be the case that getting to/from Narita would be an ordeal, but if you're willing to shell out the 3000 yen for the Keisei Skyliner, you'll be at Ueno in no time.
What time is your flight out when going back to Sydney? It might make sense to stay around Ueno or Nippori that last night so you can still do something.
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No trip to Japan, IMHO, is complete without a trip to an onsen of some kind, fake or, preferably, the real deal. Hakone is a stone's throw away from Tokyo station, and you could take a day or two there to relax (and potentially start writing some stuff down).
Last edited by KPT; Mar 31, 2012 at 6:45 am