Which island to choose?

Hiya, hope i can get some feedback from everyone!!! I will be heading over to new zealand this may and have th option to stopover in either, fiji, cook islands, samoa or tonga...how do i choose which one to go to? which one 's are not too expensive? I love the beach, swimming, snorkelling, and some outdoor activities too, hikes, water sports...but mainly relax, in my mid 20s so wouldn't mind bumping into like minded travellers...
Ummm, so many choices!!!
cheers :)

Fiji is one of the cheapest countries and the one where you are most likely to bump into other travellers of your kind.
It also has a good range of activities.

I think the Cook Islands are a good choice as well. Rarotonga is a beautiful island with lots of places to stay that won't break the bank. The people are friendly, the beaches are beautiful, the snorkeling is great, most accomodations have full kitchens, it's easy to get around via the local bus which is fun, safe, easy, clean and cheap. Check out the official tourism website! There are also some great places to stay for single travelers on a budget. Vara's is right on Muri Beach (the best swimming beach) and you'll meet alot of people your age.

sunny regards,
shully