My youngers use a aketch book, just like my older kids. I like that it will give automatic support while drawing. 😉 Another beginning resource might be the Take a Walk with series by Jane Kirkland. We were recently gifted these and I LOVE them. =) A basic resource aimed to the children. It is engaging and interesting, plus contains places for their notes and observations.
Queen Homeschool publishes a book called The Nature Study Idea Book. It is a fantastic resource for sparking wonderfully purposeful nature study, from a Christian perspective. That plus Comstock, a sketchbook, colored pencils (my girls love to sketch with a pencil and bring along watercolor pencils then use the water at home to finish their sketches), a little tote, maybe a magnifying glass, enthusiasm and you’re all set.