I wouldnt make them check the website because I sometimes people forget that kind of thing.
Anyways I will say that every motorcycle show that I have gone to there has been drawings and you can enter these to win bikes, accessories, what ever and they allways have a little thing that allows you to send them newsletters/spam whatever you wanna call it, they can opt out by checking a box or whatever. Giving a guitar strap away would be a good idea like you said.
Also make sure your business cards are readily available. If you can float it get a pic of your straps on the front of it like on your website, its more expensive but then people will be like " oh yeah that guy" or whatever.