I grabbed a radish for dinner last night, and it had new roots on it.
Look around, lots of things will grow.
Save seeds of all kinds of foods that you eat, many will grow!
As crazy as it sounds, my daughter in law ate a green pepper, she planted the seeds right out of it into a pot, and they grew! She had a harvest from the pepper she ate!
Start with just what you have on hand, dry beans and peas should grow.
Sprouting veggies normally will for sure.
Just for fun, take a celery, cut off the top branches to eat, then take the base and put it into a bowl of water, add a few drops of food coloring and watch how quickly it goes all through the plant, and the plant will grow. You can demonstrate how the plant system works taking up nutrients along with the water, in a way they will remember.
You might be able to try different colors on different days! The coloring will be temporary if you use food grade food coloring.
Talk to your plants.... If your plants talk to you... Run!