What activities will they enjoy doing? Are they active? Do they like nature or history? Maybe if you give more details on how they want to spend their time, we can offer more concrete suggestions that will match their activity goals.

Also, I just saw this thread now - your first post mentioned priceline...unless you are using the feature to choose specific flights, I would never, ever use priceline for air tickets. They are fine for hotel and rental car bookings, but never use them for air travel.

The time period you outlined is good for 2 islands - or if they only want to see a few highlights, you could squeeze in 3 - but that would add some cost.