I am a big fan of Mr. Sketch markers (and no, I am not getting paid to say that.) They work fantastically well on flip charts without the bleed through that many other brands do, they come in a number of vibrant colors, and they bring back memories for most participants.
Everytime I use them during a session, I hear a similar cry of joy, "Oh yay! Smelly markers!"
So before we use them for the first time in any given session, I ask each participant to choose a marker, I demonstrate (tongue in cheek) the proper method for uncapping said marker, and urge each participant to breathe in, deeply, the aroma. It gets a giggle every time.