I would say Santa Eulalia for couples however if you want lively nights out then that wouldn't be the place. It's quite a sedate town with little in the way of lively entertainment on a night however it does have a fantastic beach, friendly Catalan influenced culture and a great range of restaurants.

If you would like a more lively night then you may be better in somewhere like Playa Den Bossa which is home to Space Night club and the water park with some good bars however it doesn't have the young clientèle that San An has. There are some good all inclusives in the area, Thompson do some good deals in that area.

If you wanted to go to Ibiza Town then you may have to do it a little bit more DIY as the tour operators don't seem to have much of a foot hold in the capital city. It is however worth it in my opinion, you have all night markets, its home to the night clubs Pacha and El Divino, as well as loads of cosmopolitan bars inter mixed with more traditional Spanish bars serving tapas and loads of hidden restaurants can be found in the windy old town (best getting a map so you don't get lost!). It has a late night bohemian atmosphere while maintaining its Catalan charm during the day with a huge range of shopping opportunities as you might expect with a capital city. The massive night clubs Amensia and Privilge (home to Manumission) are just outside the city and easily accessible by bus (http://www.ibizabus.com) or taxi.

Hope you have a great holiday. Feel free to ask me any question, will answer where I can!