What can I plant in my garden bathtub?

Contact your County Agricultural Extension Cooperative. They are the most knowledgeable about all growing things in your local area.

I would suggest 2 things

1. See what they are selling at your local nursery since they only sell what grows and thrives in your region
2. Take a walking tour of your neighborhood with a camera to get great ideas and good photos

Flowers lots of flowers

Since you live in a territory with full sun, I recommend that you plant cacti in your bathtub. Go to your local nursery to see what selection you can buy. I live in a Chicago suburb and I have a cactus approx 4 feet tall. Inside. I'm sure you will find many more. So you could plant a variety of cacti.

I would do something simple like a drift rose and herbs. Very easy care and would look really fun.

More About Drift Roses here

https://www.gardenupgreen.com/2017/08/beautiful-coral-drift-roses.html

Then you might enjoy this read on herbs - I have many but this one will give you an idea what a nice mixture of herbs could look like planted together.

https://www.gardenupgreen.com/2016/10/grow-a-bed-of-herbs.html