If you only have 2 days in Maui, you have to make some compromise - either you spend a full day to do The Road to Hana, or only drive the North Shore.
In Oahu, you can take bus but a car would give you much more flexibility. If you search my name, you should be able to find some posts I made about Hawaii. We have visited the islands 3 times and just visited Maui and Oahu in May this year. I think I posted some info about Oahu to PPZZGG (forgot the exact name) when he asked for info couple months ago.