Giving money as a reward or award or even a present can sometimes be the most flexible gift you can offer, yet it is the least inspiring on the day.
Using a money gift certificate template from the ones shown here allows you to make a much nicer presentation and at the same time record the reason for the gift in the message. How about adding some congratulatory words and thoughts.
The fact that your voucher or certificate is free to download and printable means you can print it simply from your home or take it to a *print shop.
Make it clear if your certificate is redeemable and where it is not, if it is just for fun to make sure it is secure if it got in the wrong hands. Other than that its fully editable so you can have fun with making it.
*(Always get your print shop to verify the artwork will print to your expectation before printing)