hawaii - hikes, accomodation, tips?

Hi!
We will fly to Hawaii at the end of this month and will spend 5 days on
big Island, 5 days on Maui and 5 days on Kaui. So far we have not
really started planning - we are late, I know
Therefore I have some questions to all of you:
Has anyone got some hostels (only private room), B&Bs or cheap hotels
to recommend?
Also what hikes can you recommend (no longer than one day)
Any good beaches for snorkling and swimming?
Any other tips?

Thanks very much for your help

hdul