Hello everyone. I'm back from my trip & just rad this whole thread. Just to throw in my 2 cents worth...
I think ALL these ideas have merit.

Knowing what the issues are at the park, (weather-wise, space-wise) I think the trees & bench/or table would be appreciated immediately. As anyone whose been to a PT pic-nic knows, it gets darn hot out there and you can only pack so many dogs into the shade of just a couple trees & one or two picnic tables! And, Trees would be something the dogs themselves would appreciate. Wouldn't this be a good place to start? The "PT Memorial Grove" is a great name for an area of memorial trees.

A Memorial Board is a lovely idea, but really, it's more for the people, as most dogs can't read, LOL!

Bricks might be a good thing to suggest as the floor in that small pavillion, should that ever get off the ground. One of the problems is that the entire dog park flooded several times this past spring, I mean you couldn't even get across the big tressel bridge into the park. The little plank-bridge that crosses the little run-off creek floated way down the fence line, and the wooden palettes by the water pump ended up at the far end of the park. So the park dist. may be a little leery of putting a structure in that mini "flood zone".

So, that's it. I'll make a donation towards a tree as a memorial to Sherman, whether or not his name is on it.