I recorded three takes of improvisation / fantasy on Happy Birthday to You, dedicated to Wikidata for its fifth birthday. Above is my favorite version (take 5), below are the two other ones (take 4 and 2). (I also played two more takes, 1 and 3, but the recordings are lost due to computers.)
Each recording starts with just a reference note and then one full iteration of the melody, set in a fairly simple style – if you want, you can play just that part and sing along to it. After that comes the improvisation, which isn’t really intended to be sung.
Note that while Happy Birthday to You is in the Public Domain, the improvisation includes snippets from other, non-PD compositions, just for the fun of it. I hope I kept them all short enough to not violate copyright (and God do I hate having to think about this while I’m playing; current copyright law is trash y’all), but you might want to consider that if you want to play these pieces publicly.