Here are some more ideas to get you started.
Engage the fives senses (whenever possible)
Make leaf prints ( paint the leaf and press the print into the journal)
Observe the movement of living things surrounding you
Count the birds/squirrels/insects etc.
Measure as much of the phenomena as the students can using non standard and standard units of measurement.
Here is a downloadable lesson from John (Jack) Muir Laws and Emilie Lygren book: How to Teach Nature Journaling. All resources shared with permission. "I Notice..., I Wonder...?, It Reminds me of..."
GIVING FEEDBACK here is a fantastic resource from Jack and Emilie
NOTE: You are encourage to spend time in the outdoors Nature Journaling. However, you can also bring nature inside, look out your wonder window, have a refrigerator Safari or even observe a house plant or pet.
Begin in a standing position, keeping your feet together and your knees slightly bent, hop across the room like a bunny. Try to land softly and quietly.
Get yourself into a deep squat and place your hands on the ground between your legs. Take a deep breath and jump up, as high as you can with your feet and hands leaving the ground at the same time. Try to land in your starting position.
Start on your hands and knees. Begin by slowly lifting your knees a few inches off the ground. Take a deep breath and slowly move around by moving your right arm and left leg forward, then move your left arm and right leg forward. Continue moving around the room by moving forward with your opposite arm and leg.
Begin in a standing position and slowly bend forward to touch your toes. Walk your hands forward as far as you can until you are in a push up position. Stand up and repeat. For an extra challenge, try walking your feet towards your hands, stand back up, and move forward again first by walking your hands forward, then walk your feet towards your hands.
Begin in a squat position, keeping your knees out. Bring your arms out to the side, or bend your arms like the wings of a duck. Staying in a squat position, begin to walk around the room. How far can you get?!
Sit on the floor with your hands by your side and fingers pointed towards your feet. Push up onto your hands and feet, so you're in a table position, with your face pointing to the sky. Keep your hips high and begin by moving slowly around the room, moving the opposite arm and leg forward at the same time.
Begin on your belly. Place your arms in front of you, keeping your belly on the floor, use your arms to pull you forward, just like an army crawl.