The Skybus in Melbourne is your best option; 18 AUD each way. You can take the free hotel shuttle from there to the Hilton. It is a bit painful to take the hotel shuttle (long wait at times, and if you share with many other passengers you could be riding around for awhile). You can walk outside and grab a taxi if you aren't worried about paying a bit extra for a quicker journey.
The free tram zone is excellent, but you would be well advised to get a myki even with a small balance in case you want to go outside the CBD to suburbs like Fitzroy or St. Kilda which are well worth a look. The system does allow you to take a journey with any myki balance (it will go negative if your balance can't cover the total), so you could load a myki with $5 credit and get a round-trip out of it if you wanted to. (you end up with a myki balance of around -$2 which covers the extra cost of buying the myki).