They could mean the same thing, but they could be different. They could both mean: I would like to give you a surprise. The Past Continuous sentence would mean that it is less certain that you will actually give them a surprise. Differnt meanings could be: I am hoping ... - I would like to and it is still possible. I was hoping ... - I wanted to, but it is not possible anymore (or less likely). These 2 sentences are similar to 1st and 2nd conditionals. Like most things in English, the real meanings of the sentences would only be clear when said in context and with an appropriate tone of voice. It was a good question and you were right to think that the different tenses give different meanings. This is often the case in English - that's what makes it difficult (but interesting) :)

Tim is absolutely correct. Both could be used to mean that you intend to give a surprise. However, depending on the context, "I was hoping to give you a surprise" could mean that is no longer possible / going to happen. e.g. - "I'm sorry but I have to cancel our date tonight." - "That's a pity. I was hoping to give you a surprise."

past & present continuous... nothing more nothing less... :)

It is past and present. That's all. No other differences.