It was very funny making Dialogs with a randomly moving "OK" button responding on WM_MOUSEOVER (or something like it). Make them bigger, toolboxes, always on-top, and disable the top-right "x" button. Fun times :P
On discovering that buttons and other controls were just different classes of windows, I wrote a few little games that took advantage of it to spawn moving buttons, checkboxes, and other widgets. Their sources and binaries are probably still somewhere, but not currently on this computer. Incidentally, I believe Minesweeper also does the same with its grid --- they're just buttons.