It’s so funny because these responses show that you can’t please a wide variety of people, for different reasons! 
I would caution against (pre-ordered) sized items. The few times I did actually win something, the sizes were always inappropriate for my horse so I never got to use them! That included a gorgeous cooler and a leather halter, both I ended up giving away because after 3 different horses I realized I’d never use them.
Same goes for saddle pads - I used to always want to win one, but the irony is the one I did win doesn’t fit my horse. The cats like it as a bed, though!
My most recent swag bag had gift certificates to a local feed store (!!), gift certificate to a tack store, candy, and treats for my horse. I thought that was a great prize.
One of the events near me had gift certificates for a free BEMR session. I’m not sure I’d care for that, and agree with Texarkana that gift certificates where you have to buy in or spend $$$ to use them, are not for everyone.
I think a gift certificate to the photographer that does the show is a lovely idea.