The short and sweet answer:
Pick the prize you want to give...let's say it's the cookie. Place the cookie where you want it to appear.
Highlight the Enemy generator select new logic connection.
"WHEN ENEMY WAVE DEFEATED" option.
Connect to the Cookie and select "Reset"
Now place a level Starter and select new logic connection "When Catalyze"
Connect to the Cookie and select "hide"
Now when the wave is defeated the cookie will appear to be taken as a prize.
You also have chests that hold selectable treasures and in more complicated logic configurations you can use a replayer and make a prize out of anything.
Go to the Infinity YouTube channel for the tutorial videos by Dr Logic Gate Jose Abalos. Also visit Semerian youtube channel and Daddy Geek also has videos up on these kinds of things.
![Wink ;)](./images/smilies/icon_e_wink.gif)