10 days to do the South Pacific - which islands should we visit for beaches?

2 of us are travelling to the South Pacific in July/August. We have booked our flights in and out of Sydney and have 11 days from when we arrive. We would like to mainly sit on a beach and relax, perhaps do snorkeling one day and maybe do some hiking. Neither of us are certified for scuba diving so that is likely out of the question.

We are thinking of visiting Fiji and Tonga, however I'm wondering if that is doing too much. Main reason for choosing Fiji is that it's by far the easiest place to get to. From what I've gathered the best beaches in each country are not on the main islands, which makes doing both countries all the more difficult.

Any recommendations? I'm not against other countries even.

thanks

Rarotonga in the Cook Islands would suit your needs nicely. It is quiet, laid back with lovely stretches of beaches to lounge on. You can snorkel and there are hiking opportunities (the cross-island hike is the most popular), nice restaurants and some night life. You could also consider New Caledonia. In particular, Ile de Pins - it is the ideal of a tropical island with lovely, long, beautiful beaches. There is snorkeling, hiking and a great boat trip on Baie d'Oro. Again, a very quiet and laid back vibe - but knowing some French comes in handy.
If you decide on Fiji have a look at Nananu -i Ra Island - a couple of hours bus or taxi ride north of the airport and then a very short boat ride away from the main island - it is quiet, has the nice beaches with snorkeling and several hiking trails. Hope this helps you with your plans.

Fiji doesn't have very nice beaches, unless you go to the outer islands. Google Aitutaki (in the Cook Islands) and check out the pictures. It might be what you are looking for.

Alternatively, if you haven't been to Australia, why not just spend the entire 11 days there? There is so much to see.

To be fair, Fiji does have some truly splendid beaches in the Mamanuca and Yasawa Islands, both of which are just a short boatride from Nadi. And as OP has already noted, it is probably the cheapest place to reach (and to be in!) in the South Pacific.