Geometry Dash

Simply click on the YouTube icon on the bottom left of the title screen. You don't actually have to subscribe. Simply click on the link and when you return to the game the achievement should pop.

Edit: In the most recent patch, you now have to go to Daily, then click on Free Stuff and do it that way, otherwise it doesn't work. Credit goes to Gizea for finding this!

If you have any ideas on how I can improve this solution, I'd love to hear them! There may be something that isn't here that should be, or an easier way of doing this. I'm always open to suggestions.