OKay a trip to the garden center is in your future . Where exactly are you.? A watermelon will take up a whole huge pot , daisies are broung and only bloom for a short time. You need plants in a big pot to flower all seaon long and in FLA maye replanted for the season too I live in Canada and have huge pots all over my deck with seasonal plants for April to the end of Decemeber I change them at about Sept for some interest in the early winter . The garden center will help figure out which plants will give you continuos interest all year but be prepared for maybe a replant during the year.

I live 20 miles outside of Houston, so we should have similar challenges with heat. I have attached a picture of one of my perennial beds that has flowers that bloom from spring until we get a frost. I chose them because they are tough and come back every year. Some of my favorites would work in containers - Mystic Spires Salvia, Cone Flowers (Pow Wow series) (similar to a daisy), and New Gold Lantanas. You will have to be diligent watering container plantings in the summer, probably daily. The lantanas will trail over the sides of the container.

I would not suggest mixing vegetable with your flowers. Most veggies here start looking bad around the first of July so your planters wouldn't look very nice. Maybe you could do a separate veggie container, but as Patricia stated watermelons grow on vines and are too large for a container.

As Patricia suggested a visit to your local garden center would be helpful.