Trails are marked well, but it is recommended to take a precise description of the hike with you. Almost all hikes lead you through unpopulated areas, where you won't have the chance to buy water or food. Always have enough water with you. The humid and hot weather require you to drink a lot of fluids. Remember to wear the right shoes.
Camping is the best way to explore Hawaii on budget.
There are many well equipped camp grounds on the islands. Most of them do not have electrical hook ups and some do not provide fresh water. Kauai is famous for great camping spots. Camp in the Waimea Canyon or sleep at Polihale Beach Park. Camping permits are always required, but they are easy to obtain and cheap.