How out is Hawaii?

So here's the deal, I can get there and back for $400, but I'd have almost no gear.
How possible is it to Uber to a target or Walmart, pack a bag full of stuff, and Uber out to the wilderness and go backpacking for four days or so?
Would it be worth it to travel to other islands while I'm there?
What is the best place? I kind of want to check out mauna loa but I can't possibly climb it being so ill prepared. I think it would be cheapest to just dick around on Oahu though