Should be an easy one but I had more trouble trying to Google it than I expected. It’s the song you always hear in TV (especially cartoons) when it’s a fresh spring or someone is waking up etc.
I assume it’s something classical I just have no idea what.
Here’s me trying to whistle it. (Link goes to a site that lets you download a short .wav file. You might be able to guess what I’m talking about without even downloading it though)