What about offering them the option of flexing their hours?....IE 7-4 instead of 8-5? 10 hour days to get a three day weekend? for a week or whatever timeframe would work.
How about getting a bag of bite size $1,000,000 bars to give out with a note 'thanks a million' for your hard work'
How about taking fortune cookies and putting your own fortune on them? IE We are so fortunate to have such a great employee.
I truly think that just an honest 'thank you, we have noticed and appreciate the hard work' is one of the best rewards.