The island is tiny, so if you have a car you can really go anywhere you like. I would suggest you to avoid San Antonio, which is full of drunken brits.
If you have the chance to rent a boat don't doubt it. It's unbelieveable the huge number of hidden beaches you can find, and you can also travel to Menorca o Menorca within 3 or 4 hours (with an average vessel).
The night in Ibiza is just MAD, although I liked it more 5 years ago, with not that huge crowds, a frendier atmosphere and a nicer balearic beat. Now everything is more commercial, even MoS promotes parties at Pacha which are broadcasted by MTV. Tickets at the door of any club are really expensive, so try to buy them at the port in Eivissa.
Last advice: don't buy any pill. They are really dangerous and not funny any more. The summer of love ended almost 9 years ago.
Have fun!
P.S. If you have the chance rent a room at Hotel C'as Pla. It's a bit expensive but astonishingly romantic.