Ibiza is known as one of the most fashion places in the World. If you like sun, beach, clubbing and sightseeing Ibiza is definetely your vacation place. However not all the seasons provide you with same feelings and certainly there is a different Ibiza for every traveler.
If you like beach, sun and sightseeing but not the crowd best months to come to the island are April to the first week of June. There is few people in the island, the temperature is fine (usually 20 centigrade degrees or more) and use to be sunny. The seaside is really nice and water so transparent that you die to swim in it. In April the temperature of the water is cold but May and June starts to be ok. Also is lovely to visit inside the island. The best way is to rent a car or even better to rent a bycicle and stop to visit the small villages and make a relaxing walk through the pines' forest. If you come with family we recommend any rural hotel at the north of the island which is the - almost - untouched area. If you remain at the north of the island, even in the crowded august month, you will feel really isolated.
To come to Ibiza in Spring does not mean to rennounce to nightlife. Some discos like Pacha remain open during whole year although only during weekends. Also many restaurants are open all around the island but mainly in Ibiza capital and Sant Antoni.
Weather can change suddenly in Spring and ruin your travel with rain, storms or unexpected cold temperatures. Another drawback is that there are few planes that link Ibiza with Europe and also with Spain so you can face problems to find seat. Prices for planes can be cheaper if taken with enough time before and also hotels make offers to attract travelers. Usually to take a ticket one week before is enough to get a very nice price. Daily life is not cheaper than in summer time. Ibiza is an expensive island cause you pay to be in a fashion place and also because almost all basic things in an island must be imported and at least you pay a plus for the transport fee.
For people who likes the crowd and clubbing summer is the season. That people should remain at the south at Ibiza city, Santa Eularia des Riu o Platja d'En Bossa where beaches and clubs are really close. Another place to go to get the same is Sant Antoni at the north west. You can also be lodge anywhere in the island, at the end is not so big, but public transport is not nice in spite of the efforts of some hotels or discos to provide bus shuttles and after some days you get what is called the "island magnitude". Island magnitude means to begin to see the distances much more longer that really are just because the fact the island is small. Thus what in your country would be a normal distance to cover here in Ibiza becomes a "terrible" distance that you will get more and more lazy to make. Prices of hotels and flights are highest of the year and streets are full of people. During summer the countryside sightseeing and walk can be impossible due to the high temperatures. Summer is basically only for beaches.
September and October are still considered high season and so the prices do not drop so much. The sea water is warmer but less clean and you can find half people compared with August. During first week of October the monster clubs like Pacha, Space, Amnesia make the closing parties wich mark the end of the high season. Just for the closing parties, that use to happen on the weekends, many people comes from Spain and all around Europe so the island gets crowded for the last time. After the closing parties many clubs remain closed till summer, others open just for the new year eve and Pacha keeps open only on the weekends.
October and November are once again nice time for rural hotels and enjoy of the last sea baths. In december life becomes lazy and just new years eve brings some bright to the island. January, February and March can be hold only thanks to some clubs and the rural hotels.