You might find some ideas from this site: https://raisinglittleshoots.com/2015/08/17/supplies-for-nature-journaling/
She mentions water brushes. These are brushes that you put water in the barrel, and you give it a little squeeze. That way you don’t need to carry a separate water jar.
Some pencils and erasers might be good to sketch the design first, to get it as desired.
Then crayons or colored pencils or watercolors.
It’s a nice idea to sit on a stump or a stool and sketch, when it’s nice weather, for more fresh air and day light.