Greetings,
I need some help from the veggie experts!

My friend is on the gardenwalk, July 12 in Wisconsin.
So far, we are having a very cool/wet spring and forecasts say it will be a cool summer overall.

She has a big, antique manure spreader (like an old wagon on wheels) they she'd like to stage with large leaves flowing out of it.

We thought that melons/pumpkins/squashes could be the main player....but we need your opinions and advice. We also thought veggies like red kale, swiss chard, could do well considering the cool temps...they could be placed higher, at the back. (She can place pots at different levels in the spreader.)

Here's the idea: plant large (7-10 gallon) pots with vining vegetables and cool weather veggies. Place them in the sunniest possible location for as long as possible, like on a driveway for warmth....then put them in the wagon about 10 days before the event, or sooner if they are growing well and getting too big for easy moving.

More thoughts we are considering:
seed the cool weather veggies ourselves right now (kale/chard, etc)
buy the vining squashes/melons from someone who has a warm greenhouse, perhaps in late May

I'd like to know if this could work (the vines in the pots) and what might be good strategies for common cultivar selection, etc...

Really, any thoughts that could contribute to success would be welcome. We are not going to put all of our eggs in one basket, so to speak, so if you have any plant/seed/cultivar selections that would work for a cool summer, we're happy to hear it.....we are going to try and get castor bean seeds, and maybe some taro roots going, as well.

For this project, we are not at all concerned about a harvest, we just need the plants to look good on July 12.