I love Ibiza, I go every season =D
For things to do, Formentera is amazing, seeing Es VedrÓ from the pirate tower, sunset strip sun set (Cafe Mambo etc), Dalt Vila, then check out some of the sneaky beaches on the far end of the island.
If you're looking for somewhere to get a hotel I'd only recommend Ibiza Town, Playa Den Bossa, or San Antonio. That is unless you don't mind being raped by taxi's all week long farting around back and forth on the island, which can be BAD.
However, Ibiza is for partying, and if you do it right, that shit is tough to get in. My personal favorite club is Amnesia, but all the big ones are gonna be fun.
Amnesia has Cocoon on Monday which is Sven Vath and those guys, Hyte is a new party on Wednesday which host awesome parties in Europe, Cream is your trance night, but they haven't announced the days for that this year yet, and Music On is on Friday, which is Marco Carola and a bunch of coked out Italians that party until 2pm in the terrace.
Space has a few big nights with Richie Hawtin Enter every Thursday, and Carl Cox on Tuesdays are good nights musically.
DC10 is awesome, but it's a more gritty feel to it and it only hosts two nights, both are gonna be house variety.
Ushuaia is a day time outdoor venue, which is pretty nice, but they have mostly cheesy EDM short of Ants on Saturday, and Armin van Buuren has ASOT parties there every week, but the day for that hasn't been announced yet.
Sankey's has more of a standard club feel to it, it's basically all English people, but hosts to some good nights like Tribal Sessions and Viva Warriors.
Pacha has a couple good nights like Solomun +1, but it's mostly EDM.
Privilege is the biggest club in the world, but the nights there are pretty lacking, once in awhile they have a one off great night and pack the place out. I think the first weekend of August is BBC radio 1 weekend in which they host a huge party that sells out with 15,000+ people.
If you like trance, definitely hit Driftwood boat party. It's a sunset boat party out of San Antonio.
Check out these forums for more general info, I could type here all day, but if you have specific questions you can PM me.
Ibiza Spotlight Forums
Here for a list of parties, it will fill as the season comes closer.