1. Plant flowers together and measure growth on a special chart. It you don’t live close, send a seed packet to your grandchild and both of you plant at the same time. Plan to call or email once a week to compare progress. This will be interesting if there are two different climates.
2. Make a card or design with pressed flowers from your garden. To preserve, iron the slower between waxed paper and cut out.
3. Plant something in a clear plastic cup so you can see the root growth. Plant herbs in egg cartons filled with cotton.
4. Decorate clay pots with buttons, ribbons, paint, etc to hold a special plant
5. Write a story about what you planted in a pot or in the garden. Flowers and trees can be the characters. Use paper with a special gardening border available at stores that sell scrap booking supplies.
6. Paint or decorate a picket fence to go around the garden. If you planted something in a pot for inside, you can use a cardboard box or poster board to make a fence around the container.
7. Decorate a small watering can and fill with gardening items and book to give as a gift to your grandchild.
8. Plant a tree or bush in honor of someone or something. Write a notes and bury it with the roots.
9. Plant a tree or bush and have your grandchild make a goal to go along with the growth of the tree. Keep track of the growth of the tree and your goal.
10. Using craft sticks, paint and decorate markers for your vegetable garden. Write the date they were planted and by whom.

There are also many children's books about gardening and plants, Miss Rumphius being one of the more popular ones.

A good plant for measuring is a sunflower, especially the mammonth ones which get very tall and grow pretty fast. If you have room, you can plant sunflowers in a circle, and when they grow up, they make a sunflower house the kids can go in. I read that somewhere, haven't tried it.