We have been to the 4 main islands in the past 3 years, and my favorite is Maui. Oahu is second, and the Big Island (Kona side) is third. I was not a fan of Kauai, though the island is probably the most visually stunning.
Oahu: This island is very busy, especially if you stay in Waikiki. I personally liked the city feel of Honolulu, but I don't think most people appreciate that on their island vacation. The north shore is much quieter and very beautiful. Like others have mentioned, there are a lot of touristy things to do on Oahu. Seeing Oahu from the top of Diamond Head was really neat.
Maui: Extremely laid back island, which is why I like it. You can stop off the side of the road at virtually any beach and get in the water, which is not the case on Kauai (many of the beaches are unswimmable). There are some great snorkeling spots on maui - black rock (we saw some beautiful rays, and TONS of turtles), kapalua and honolua bay, ahihi-kinau reserve. There are also great hiking spots both forest and desert. I'm not a fan of the road to hana (mostly because I don't like being in cars to begin with, and the road can get pretty crazy with the one lane sections) but most people that visit Maui like to do that trip. We have stayed here which in my opinion is a perfect spot and they have a great cabana right at the beach:
Outrigger Maui Eldorado - each unit has a full kitchen, and I'm pretty sure they have units that sleep 5. Their prices have been very reasonable when we stayed, and you won't have to worry about the vrbo stuff.
ETA: You will be able to surf on Maui in June. We have gone around the same time, and my husband purchased a cheap surfboard on craigslist and had a lot of fun surfing. Obviously the waves aren't huge but good enough for someone who is not a pro surfer. You will likely not see any whales as the winter is when the whales head to Hawaii. We have found it cheaper to book plane and hotel together, we've used both travelocity and beachdestinations.com and got great rates. If you do some cooking in room there is no reason you would have to spend $250 a day to eat well.
Kauai: Like I mentioned, many of the beaches in Kauai are not swimmable, so the ones that are are usually packed. There are some nice hikes and the scenery is probably the most beautiful. But the locals seem pretty hostile toward visitors. That and the swimming are the two main reasons we won't be back to visit.
Big Island: This is a really neat island because you can see how the lava flowed from the volcano and created the landscape. Everything is really slow and pretty quiet, but we found this island to be warmer than all of the other islands, we were constantly in search of shade, which was hard to find. We found the best snorkeling on all the islands here, at captain cook's monument. You will either have to pay to canoe over, or hike down the mountain which is pretty rough, getting back up after snorkeling for a few hours was the hardest part, but doable if you are in reasonable shape. Many people will suggest to you to stay a few nights on the volcano side of the island. We booked a small cabin at airbnb and stayed one night after we spent the day hiking the volcano. We also drove around and hiked to the green sand beach on our way back, which was really lovely. Everything on this island is really spread out, so be prepared to drive a lot, and try to make plans (activities/eating/shopping) based on that.
