Am thinking about hitting up an island on my way to NZ in January 2008. I'm looking for your suggestions and advice on where to go...and where not to go....
Who: female travellers in their early 20's
What: looking for a beach holiday in an inexpensive and clean accodomation very close or on a beach on one of the islands in around New Zealand (thinking about the cook islands). While we like to have a good time we have no interest in being surrounded by lots of crowds, amboxious beer guzzler-types, you know? Looking for something quiet but also close enough to a place with a few local cafes or small bars
when: January 2008
why: looking to relax, check out see some cool flora and fauna, stroll (walk) and read
how: air travel

How about Australia - it's an island and much close to New Zealand than the Cook Islands. Or alternatively, you could look at the islands of <a target=new class=ftalternatingbarlinklarge href="http://www.aucklandnz.com/VisitorInformation/AucklandGuide/hauraki_islands.aspx">Hauraki Gulf, much closer</a>.
The Cook Islands might do the trick - the main town has one very good cafe, some average ones and a few bars. Most of the decent cheap accomodation seems however to be strung out around the island - I stayed on the south coast and while very relaxing, I had to go into town every day to get my fix of good coffee. I did manage to knock off Ulysses in my week there. But a big consideration will be where you're coming from and how accessible the particular island you choose is - I think there is one Air NZ flight originating in the USA that comes in through the Cooks. Tahiti is much easier, as is Fiji (if you don't mind a coup and a deteriorating political situation.

Why don't you investigate Waiheke Island in New Zealand - it is an hour or so on the ferry from Auckland, has gorgeous beaches, lots of places to go for walks, native bush, very quiet, yet in the main village there are nice cafes, restaurants, bars etc. There are also some really good vineyards on the island, I'm pretty such you can do wine tasting tours. Often in summer various New Zealand bands do gigs over there which are usually quite good to go to. Its got a really good vibe, lots of young people around. There are some good hostels to stay at too, or else you could consider hiring a beach-house which can work out pretty cheap if there are a few of you. Of all the islands in the Hauraki Gulf it is the most accessible, and also has the most going on, some of them are pretty isolated.
If you were thinking more of a tropical island Fiji is quite close, about 4 hours on a plane from Auckland, and I think it is cheap at the moment due to some politic unrest - stick to the Southern Islands. I can recommend Beachcomber Island and Plantation Island, I went with a group of friends to Fiji a few years ago and we stayed on Beachcomber, it was NZ$77 a night including 3 meals, accomodation (dormitory style) and daily activities like snorkelling, fish feeding etc. We relaxed on the beach, went kayaking and snorkelling, swam lots, got massages, and at night there is a cool bar there...
But be aware that January is school holidays in New Zealand so wherever you go, including the Cook Islands, Fiji, and any beach place in New Zealand it is peak season, so pre-book accomodation etc, and also there will be lots of families around.