Agreed on the parks thing. I have a feeling that will be a no-go no matter how we look at it, so we'll need to look at a private facility (such as a house).
Indybeer raises a good point regarding the sixers. In my head, we're not sampling every single beer. We're sampling 6-10 different beers or so across a selection of 20-30 different flavors. I assume there will be plenty of leftover. I only said "six pack" because that's the smallest unit available and we won't have a dozen beers per person.
As I'm sure most of you who are into craft/import beers already know, 22oz. bombers are pretty common sizes, and offer a pretty decent amount of beer, especially for tastings. They're typically a little more expensive per bottle, but might allow for a little broader range of beers than 6-packs.
Also, I suggest we get together a list of items for the tasting, like someone already mentioned snacks. Those little plastic cups are great for tastings too.