Pair with {{Show bottom}}.
label — text of the collapsed button (default Show).
wyg — what you got. Set it (wyg=1) to park a little Cromulon head to the left of the button; hovering it says Show me what you got. Off unless asked for; wyg=0 and wyg= are also off.
The showbox class (see MediaWiki:Common.css) keeps the button on its own line with the content flushed below at full width; showbox-wyg puts the head and the button side by side.
{{show top|wyg=1|label=Show me what you got}}
…the hidden stuff…
{{show bottom}}