How about telling them to bring some old boots or shoes and planting things in them that they could watch grow at home and think of you. Red rubber boots or yellow are my favorite or maybe putting some annuals in them so they look grown into.
Just a thoughtor make some stepping stones with their handprints and painting them(hpertufa has recipes)just use pie pans or something.
debbie

You could plant some thing in the clear plastic cups for them to take home with them. Then they could watch the roots develop & possible transplant them. Marigolds are good & easy for this.

Get some clear jars with lids and make terrariums.

If you are looking for indoor activities, you might go to a craft store and see if there is anything they would like to do. Or just buy them ahead. I saw a little girl in a store with some kind of velvet color or paint by numbers, and boys usually like models.

The purpose of my search today was to find ideas for our kids club at the YWCA to do.
Catrina

Ideas for Kids Club

o Heads that grow grass(put grass seed in a kneehigh stocking and fill with soil, tie it off and place in a cup to absorb water add a face.
o Fuzzy Grass catipiller- similar to the above idea except you put the soil all along the stocking and tie it in segments lay it in a flat dish and add eyes and antenni. Maybe a little more difficult as you have to make sure the seed stays along the whole side of the stocking.
o Chia pet
o Bird walk in spring
o Make bird feeders
o Plant something in a boot or shoe.
o Stuff to grow-I know you have done the sweetpotatoes, how about;
Pineapple tops
Gratefruit seeds(or other citris)
Mimosa Pudica (sensitive plant)or Partridge pea (chamaecrist fascicelata)
Avacados
Teddy bear or baby sun sunflowers grown in a pot.
o Paint a pot or garden marker
o Blind walk(or have things to feel while blindfolded)
o Nature memory game-Gather natural objects from the immediate area. Have them under a cloth. Let the kids look at them for 30 sec then cover them and have them find another just like them eg. Seeds, rocks, cones leaves
o Microhike-give them each a magnifying glass and have them all find things that are smaller than a cherio
o Noahs ark-(may not be enough kids for this) Have cards made with animals, 2 of each kind. Each kid draws one and keeps it secret. Then they act like their animals and try to find the other animal that is like them so they can get on the boat(have a place designated that is the boat.
o fairy gardens
o Bean teepee
o fall preserved leaves
http://www.conservation.state.mo.us/nathis/seasons/fall/preserve/