According to Wikipedia, Emily Post says you should not give a gift to your boss unless it is from a group of employees. Nobody likes a suck up.
Humor is a great way to show appreciation while having fun at the expense of your boss. Just make sure they actually have a sense of humor first.
Below are some free printable certificates you can present to your boss. Just download the free certificate, fill in your boss's information, and print. Within seconds you'll have a gift idea that's fun, free, and easy:
Raisin Pay AwardFor the Boss Most Deserving of a "Raisin" Pay. Download this free Certificate of Appreciation
Loch Ness AwardFor the Boss who is Least Likely to be Found. Download this free Free Printable Award
Running with the Bulls Leadership AwardFor the Boss who is the Biggest Risk Taker. Download this Free Printable Certificate
If you need serious gift ideas for Boss's Day, cards, flowers, or balloons are a good bet. Or go in with your coworkers to get a gift from the entire office.
Just don't forget National Boss's Day 2010. After all, your boss is... well, your boss.