Post in here if you plan to make the trip up, when you think you will head up and when you will come back. Probably not a silly idea to look at booking ferry spots now so we don't miss out later or get stuck with bad crossing times.
So the more info you can provide the better.
I haven't worked out when to go either yet, but possibly the below: (prices will vary depending on company)
Trip up - finishing at lunch time friday
Friday depart Piction: 6.05pm (arrive 9.15pm) $245 car + 2 people
Return trip - take the extra day off Tuesday
Tuesday depart Wellington 8.25am (arrive 11.35am) $220 car + 2 people
Thoughts? Suggestions?