There are a bunch of easy plants.
What is your substrate now?
You could change things this weekend if you wanted.
You can use sand, like pool filter sand or construction sand. May end up with diatoms and will need to rinse them pretty well.
Eco complete is a good basic substrate, you can get the 20lb package from petco for $22.06 as of today, and buy online and save 10% to pick up today in store. (if it is in stock) Probably other good options online and maybe in your lfs
for 4th of July sales. Even cheaper on amazon with Prime but you wait for it.
What kind of minnows do you have?