Generally you have three option to go to to Abu Dhabi either by plane this is the fastest and most expensive option, second option rent car or limo this intermediate in cost and in time , third and most practical and reasonable option is to use bus from Ghubaiba bus station near metro to the very central Abu Dhabi bus station but this will need you to wait the bus for a while .
Regard to my recommendation about things to do in A bu Dhabi first visit ferari, aqua park, and Louver museum.