I've done this in a few small groups, using:

• the "this is an attention test" video (aka "how many passes does the team in white make") - always a hit
• a calibration exercise just as Larks suggests below, which works pretty well
• the Wason card task - some people find it unconvincing "because it's logic" (!)
• a "learned helplessness" experiment using impossible anagrams - works so-so
• a "priming" experiment where two groups have a phrase-building task with different priming words - my version of that sucks, need to rework it
• I also show the Spinning Dancer often