Heading to Oahu and staying at a hotel on Waikiki beach. Is getting around the island easy? Or should we get a rental car? Any suggestions?

Getting around the island on the bus is easy, but not convenient at all. It's best to rent a car. That way you can stop and take in the view any time you'd like.

You likely won't need a car for the entire time you are there. I would pick up a rental car for a day or 2 to drive around the island etc. But otherwise, within Honolulu the bus runs frequently enough that it is unecessary to have a car and find and pay for parking etc.

You can hop the Waikiki Trolley for touring Honolulu & Waikiki:
http://www.waikikitrolley.com/

And O`ahu's bus system covers the whole island -- cheap:
http://www.thebus.org/

There are free shuttles to places like Ala Moana Shopping Center, Hilo Hattie, etc. (ask your hotel for pickup info).

As Miramar has pointed out, renting a car for just one day -- if you really feel you need one -- would probably be a better plan.

Was recently in Waikiki for 8 days without a car, which was fine for me. But I was doing things only in Honolulu area.

There are many local car rental lots with cheap deals posted. Just depends on what you want to do if you'd need one or not.