Have them plant a garden outside, if possible. But what about inside too? I facilitate a playgroup for children ages 0-5 on a Indian reservation and have done this activity a few times.

I bring a hard plastic swimming pool, then pour shredded paper into it (from our office's shredder) & have the children make plant markers: tape laminated seed packets to popsicle sticks. I went on "Print Shop" to run off copies of seed packets and then laminated them. We'll also bury pretend (plastic) vegetables under the paper and stick the "plant marker" over each vegetable. You get sensory and fine motor stimulation with this activity.

Hope this gives you an idea. Of course you can grow the "bean inside the baggie" also. LOL.

Get each child one of those small milk cartons that are used at school. Cut the top off, add dirt, put two bean or corn seeds into it, add water. Put each container into a large zip lock type bag and seal it. Place them on the window seal. You will not have to water them again until you take the plant out of the bag. You do this when the plant hits the top of the plastic on the inside.

Go outside and use growing things to illustrate shapes, find signs of spring/fall, whatever. Growing things is great but if they are young, just use the outside to explore. THen grow something like radishes (fast) and discuss the different parts of plants, etc. Think outside the box. Have fun!