Thanks for all the advice.
If I weren't to buy a new lens, and just go with the kit lens, would the shots still be pretty great?
If I did this, what filter(s) should I get (I'm already going to order a CPF)...
You guys have really helped me out!
you have to try really, really, really hard to not take a good shot in Hawaii. What you have will definitely work. i basically split half my shooting between my 18-200 3.5-5.6 and my 50 1.8. the 50 1.8 really came in handy when i went to the old lahaina lua. we had front row seats, and i didn't need the flash at all.
i do recommend, however, going to mt haleakala before noon and be sure to stop on the way up and down if you plan to go. I went later than noon and the trade winds were starting to blow over the caldera, had we waited longer i'd have gotten few shots. used my 18-200 that whole adventure.