AppleScript to put your Mac to sleep after X minutes, fading out the system volume shortly before sleeping. Can be turned into an app via the "Export" feature of the AppleScript Editor.

sleep-timer.scpt

-- written by Florian Bender, 2015
-- based on work by Michael Wyszomierski <https://wysz.com/wyszdom/2009/06/simple-sleep-timer-with-applescript/>
-- license: Public Domain / CC0 <http://creativecommons.org/publicdomain/zero/1.0/>
tell application "System Events"
	set volumeSettings to get volume settings
	set volumeValue to (output volume of volumeSettings)
	set fadeTime to 10 -- time in seconds for fade change
	set fadeChange to (volumeValue div fadeTime)
	-- display alert "Volume: " & volumeValue & ", Delta: " & fadeChange
	display dialog "Sleep time (min):" default answer "25"
	delay ((text returned of the result) * 60 - 30)
	display notification "Going to sleep in 30 sec" with title "Sleep Timer"
	delay (30 - fadeTime)
	repeat with X from 1 to fadeTime
		set volumeValue to (volumeValue - fadeChange)
		set volume output volume volumeValue
		delay 1
	end repeat
	sleep -- alternatively: do shell script "pmset sleepnow"
end tell