For occasional "special" gigs (i.e. where we get paid more than normal) we bought a dozen or two cowbells like this on Amazon and passed them out to the crowd immediately before we play "Honky Tonk Women". The bells are $1 each and barely last the song, which is perfect.
Audience reaction has been great, but we also wait until everyone's liquored up. Also, our drummer's own cowbell is miked, so we can turn his up to avoid the inevitable poor timing that the crowd introduces. We really play it up - have the audience "practice", really ham up the demonstration, etc.
Alternatively, we've done the same thing with just a single audience member - if it's their birthday, reason for the gig, etc.