There are so many possibilities... I wouldn't feel comfortable making suggestions yet, though. What colors do you like? What kind of look do you want... Crazy busy with plants bursting out in all directions? Upright, more formal? A grouping of plants that will bloom at different times or at the same time? Interesting foliage? The combination of full sun, hot bricks, and humidity from the waterfalls sounds like good conditions for a lot of plants. Did you want the 3 sunny ones to be identical? Are you trying for a look where all 4 are similar although one is in different conditions?

Morning glories are easy from seed but are vines and usually used where there is something for them to grow on. Are you visualizing them spilling over the sides?

I'm also curious what the planters are made of and why you mention lining them with plastic?

The link below is really cool. I think it's easiest if you zoom in on your area some first, 3-4 clicks. It could take a few seconds after you click a box for the color to show up on the map. You are probably in 6b or 7a.

Here is a link that might be useful: You can find your zone here