I haven't seen too many episodes of HIMYM. But, I know the two main characters date A LOT. This creates the perfect opportunity for cameos from attractive stars. You'll love this gallery from MadeMan: 25 Hottest 'How I Met Your Mother' Guest Stars.
Here's one to get you started. Stacey Keibler played a bartender who only started dating Barney, because he wasn't wearing a suit. On their first date, Barney finds out she hates men in suits. He likes the girl, so he starts lying.