You could go for the whole 'Love Actually' idea and have actors/musicians hidden amongst your guests or the waiting staff? They then wait until a certain point and break into song. There's a company called Incognito (
http://incognitoartists.com/ ) who specialise in that kind of thing.
Or you could have a hog roast? Firebreathers are a great idea, you could go for contortionists as well for a circus theme. Or a burlesque act if your guests won't be offended!