I think there are only a few rooms that accommodate couples so it would be better to book now.
Also plan on arriving earlier to enjoy the hydrotherapy gardens.
We were there mid-September in 2016 and did the couples massage. I booked about 4 months in advance and had no issue changing the time about 3 months in advance. I also recommend booking now so that you can get your choice of day and time. Our service was at 11am and we used the hydrotherapy gardens before. We then had the rest of the afternoon to enjoy the resort!
If you are talking couples treatments, if your budget allows, I say go for one of the couples spa packages. It includes more than one treatment and is soooo relaxing and luxurious! But a couples massage by itself is luxurious too!
Laniwai Spa has a very nice menu of offering very much like any high end resort spa....facials, massage, body treatments, pedicures, manicures, etc. And all sorts of options for each category! I recommend you take a look online of their spa offerings. That will give you a very good idea of what you would like to book. And if you have any questions about it after that, just give them a call! Happy planning!