Planning a trip to Hawaii

I'm planning a trip to Hawaii for me (30) and my boyfriend (35) for 5-6 days sometime this summer. I've been hesitant on which island to book on. Originally I wanted to book on the Big Island...spend two days on the rainforest/volcano side to do some hiking and adventuring then make our way to the west side for R&R and drinking/nights out. Looking further into it, I'm just unsure if the Big Island offers any "nightlife" type feel that perhaps Oahu has. What would be your recommendation for a split trip as far as half adventure/hiking/exploring and nice beaches/resort/nightlife?