He or She is a Boss and should be celebrated…
So first of all let me say that a card, even a group card, will do just fine.
But keep in mind that if your boss is great (and not everyone is so lucky), you may want to go a little bigger.
So keep it nice and workplace appropriate by treating your boss to coffee (or tea, or whatever their morning beverage of choice will be) at his or her favorite retail cafe via gift card. Throw that gift card in a new mug and you’ve got a winner.
If you’re going in on a group gift, I suggest that you make it a group celebration and do what I affectionately term a “Bagel Breakfast”. Get $2 from every member of your team, buy a box of bagels and a vat of coffee, and treat your boss to a surprise office party in his or her honor.