Say they were giving away* wristwatches?
In fact smart wristwatches with Bluetooth?
Well, not really giving them away, but...
What happens to the device at the end of the event?
2 flavours - a simpler version of the device intended for re-use at future events - or a fancier version with an LCD screen, and a wirelessly rechargeable battery.
The device has a future life as a smart watch (or even just as a wristwatch) that connects to any modern smartphone to provide email/text/missed-call alerts (or it can just tell the time!).
One distribution model is "Lucky dip" at live events - your device is boxed, and you might get a smart watch and you might not. This lowers the costs, but still creates the pool of fortunate users (***and*** you do the upsell to those that aren't so lucky).
Alternatively, the more sophisticated version is presented purely as an "up-sell" at events where the attendee can trade-up their basic device, at modest cost.
Or even, for appropriate events, the premium device may be a branded souvenir included as part of the event.
(*however- also read the next section about post-event engagement)