In our trips we have done the 4 main islands but never more than 2 per trip.
Flights from London generally via LA or San Francisco so if you would like to visit either of these you can on the way out or back. Flight costs average out at £1000 in economy during the year. Leaving London in the morning with 12 hours to California then around a 2 hour layover then another 5 hours to Hawaii you arrive that evening, but be aware this does mean you are driving tired. If you stop on the way out you can get a morning flight to Hawaii and arrive for lunch.
My favourite island is the Big Island, for the dramatic volcanic scenery, active volcano, laid back style, fantastic beaches and lack of people. But some people hate it. To explore this island you need a minimum of a week it is called big for a reason and I would recommend staying in the Waikoloa area. Many people opt for a night on the other side of the island as the drive to the volacano area is long, we have never bothered but DH likes driving and is very experience in driving in the US. This is the island for exploring.
Oahu is the busy, built up party island again IMO I would spend less than a week here, to experience Waikiki beach, go to Pearl Harbour and the malls. It has a completely different feel to big island and they make a very good combination. Or possibly my favourite which is Oahu 4 days, Big Island 7 days, San Francisco 3 days.
Kauai is beautiful and green with good beaches and chickens but I have been once as I much prefer BIg Island. It is more built up and doesn't have the dramatic new volcanic landscape.
Maui is the resort island, and as PP have said it is the perfect honeymoon island, it is a good balance between Oahu and Big Island with good beaches and scenery but a more vibrant yet still laid back style. This is my go to island for a week or so of Rest and Relaxation.
On all the islands there is plenty to do, snorkelling, paddle boarding, trips to volacanoes, coffee farms, movie sight seeing trips, helicopter rides.
My final piece of advice would be not to try to do it all in 2 weeks as you will feel you have been cheated as each time you start to get to know an island it's time to leave.