Laganas in Zante. Perfect. Cheap, fun, fantastic nightlife, McDonalds(!!), lovely people, basically the new Faliraki. If you want history Zante town has it. There was an earthquake in 1953 that flattened all of the island except for the church in the centre of the town, this also means that every building is no more than 51 years old! Weather is good during the summer months until about the beginning of October.
If not there try Faliraki. It is going cheap this year becuase it is quite quiet out there due to all the bad press. I think this kinda adds to it though. There is a fantastic beach, shed loads of bars/clubs, great people and again a McDonalds for anyone who won't eat if it isn't American! The weather is good too as it is breezy which takes the heat out of the sun. Again sunny until about October.
Or Kos town or Kadamena. All very lively places, probably get a good deal if you wanted to go last minute.
Malia in Crete I've been told mixed reviews about but I wouldn't mind giving it a go! Probably the same kinda thing as the rest of the Greek Islands. And Corfu is fantastic and also has a bit of history if you go into the town centre.
Ibiza probably isn't the best place to go to. Unless you and your friends have really good jobs. I went when I was 17 and you can't really afford to go to the big clubs unless you've been saving for months, a bottle of water can cost �6! But if you stick to the west end in San Antonio you can't really go wrong, free shots in every bar etc.
Or Playa de las Americas (wrong spelling sorry!) is quite good too I think that's in Tenerife.
I'd go into your travel agents and ask about prices and stuff and see what they recommend. Remember you aren't obliged to book anything if you do just go in for a chat! Good luck and let us know how you get on!