[Ibiza]

Since I've been living in Split and Zagreb, I'll recommend Zagreb. Any other city is much smaller. Split is also nice if you love slower pace of life and warm weather, there are lots of foreigners moving into Split and they have their own community. But it can get boring during a winter.
There is much more activities to chose from in Zagreb. I recommend renting an apartment in the center, it is not expensive (since there are mostly old buildings, but still, nice place can be found for the amount you mentioned and have it for yourself). Public transportation is great (much better than in Split), so you can get fast in any part.