A very different type of tune has caught on for Six Flags. The old-dancing-guy character created for the amusement-park chain's "It's Playtime!" campaign has several blogs dedicated to him, with much chatter about his age and identity. (Neither Six Flags nor its ad agency, Doner, Chicago, will say who he is, or whether he's a young person made up to look like an octogenarian.) He's set to appear on ABC's "Good Morning America" this week, and merchandise is selling out at the parks. The ads have reignited interest in the catchy background song, "We Like To Party," by pop group Vengaboys. The tune, released on independent label Strictly Rhythm in `99, is the top downloaded song in Atlanta, and is now played at Atlanta Braves games for homeruns. "We wanted the campaign to break through," says Six Flags spokeswoman Debbie Nauser, "and it definitely has done that. We're just delighted."