Chinese equivalent of 'Surprise!'
I can't recall any Chinese expressions used in the same way as calling out with 'Surprise!' in English. I guess the reason might be that Chinese Culture doesn't make Chinese people as playful as English Culture making its people.

We say something different from 'surprise!' in similar cases:


I bring a gift to a friend, before showing him/her the gift, I say: 猜我给你带来了什么？or 看我给你带来了什么？ Then show the gift, and after seeing his/her facial surprise expression, I say: 没想到吧？or 喜欢吗？He or she may say: 真没想到。or 我很喜欢。or 我太惊讶了！
We plan to throw a party for a friend's birthday, and one person leads the friend to a room where everybody else is hiding. When they come in, all the hiding people jump out together and say: 生日快乐！
In a classmates reunion party, we decide to invite the old flame of one of our classmates. Once that classmate arrives, we say to him/her: 你看谁来了？Then the old flame comes in to give him/her a surprise.
At a performance audition, the judge is pleased with an actor's performance. The judge may say: 你的表演很不错，但是你知道，其他的表演者也非常优秀。我知道你非常热爱表演，所以不论今天你是否被录取，我希望你一直坚持下去，不要放弃。Now the actor probably thinks he will not be picked. Then the judge says: 恭喜你，你被录取了！The actor is definitely surprised.