I don't have a template. I usually just make a table in MS Word, type in what I want the coupon to say, add clip art, and print.
But - I always use a barcode font to put a cute barcode on my coupons - just like the real thing! The kids love to read the tiny words at the bottom of the barcode. You can probably find it for free if you google "barcode font". I'm sure I downloaded mine free..
This is what I give as a Christmas coupon every year. Last year I got smart and put all the disclaimers on the back of them, but I apparently didn't save the pages that I printed on the back of them in the same file. I use a business card template in Word. I hope this helps.
I've always created my own coupons using PowerPoint. It allows you all the border, fonts, etc. and it allows you to move things around more easily than with Word. You can also manipulate graphics more easily - changing size or placement on the page. Inserting whatever you want is a breeze. Then you just print the slide!
I use Power Point for almost all my worksheets now too. So much easier to manipulate text boxes around the page!