Bring a camera, Hilo
Luggage and bags:
You're better off not bringing loads of clothing here as you'll probably be shopping for it anyway. The good news is, you need not bring cold weather gear as it's pretty comfortable weather wise. Unless you are traveling with family or staying long term, travel light in the luggage department.
Clothing/Shoes/Weather Gear: Shoes are a must, preferably tennis shoes if you're going hiking in Volcano just outside of Hilo. Beach wear is just as you think, skimpy and comfortable is good enough (for our Euro tourists, please no speedos!!!) We're not conservative but "being on display" is a bit over the top :-/
Toiletries and Medical Supplies: Nothing more than a couple of bandaids will do... If you go swimming and get stung by a jellyfish, see a lifeguard right away.
Photo Equipment: Take a camera where ever you go. This is Hawaii for goodness sakes! You may want to invest in an underwater camera to get some nice shots especially if you're going diving.
Camping/Beach/Outdoor Gear: Camping... on the beach? Don't forget your ghostbusters kit muahahaha. Depends where you go, you may run into some nasty insects (centipedes, mosquitos, cane spiders). Bring a spray on bug repelent.
Miscellaneous: Bring money, all you'll need really. You can buy everything else.