With Booth 2.5.568 or above you can have the software select a special winner to receive a prize. This option is located in the man tab.
Manual - This option allows you to press a button in Booth Control to force a winner. Just press while in the session and that will produce the winner.
Random - This option lets the software choose the winner based on a number you set. You can also set the max wins per hour or day. so you can have several winners or just 1. You can also enter text to be displayed on screen for the winner.
Options to add sound video or device control for lights etc. that will open at the end of the winner session. You can also set a special template to print in addition to or instead of the standard one. This can be used as a coupon for the winner.
Here is a free sample event to help you get started.