If you have air conditioning they should be fine as far as heat goes...but if I had to guess I would say that romain lettuce, kale, and maybe some watermelon...I knwo there's hardly any nutrition value in it for them, but it'll be high in water!Oh, and cucumbers! They love those and they are high in water I'm sure!
Ok, I just looked at the GL page and they had water content on one of the nutritional pages and all of the above are good, plus watercress (?)...don't know how available that is where you are, I have never heard of it! lol
Have fun and show us lots of pics!!