Do you have any volunteers who are also photographers (or even just do it as a hobby?) You can offer professional pet portraits as part of your event. I think you can set the donation from anywhere between $25-$50 depending on what the photographer is willing to do ie will he or she provide all the photos, do any retouching? Since you already have a lot of props, you can add those to the photos if they'd like. Hopefully this will bring out people that already have pets and aren't looking to adopt.
You can also see if the pet store you're having it at is willing to offer you a % of sales. You can use that in your promotion, too.
Do you have any ewok-y looking dogs right now? You can use them in the promo materials.
Can you offer a reduced adoption fee for cats and dogs older than 4?
#MarketingandSocialMedia