Get into the wrong level, and collect melons there. Once you do that, that level’s melons will “glitch” causing them to respawn an extra layer of melons each try of a level. If you do it more than one times on a separate set of lives, you can stack this effect where it spawns TWO layers of melons each attempt at a level. You can also attempt a level two times before moving on, so dying the first attempt to get a second results in this.
You ever wanted to collect a lot of melons? Well, I guess you can now.