Cut the tops off of enough milk jugs for each preschooler. The jugs can be different sizes and shapes, but be aware that this may lead to a fight, so it's probably best to make them all the same or have several of each type. Let your preschoolers decorate the exterior of the jugs with washable paint, markers, glitter glue, stickers or other suitable materials. Fill the jugs yourself with soil from a bag, or take the kids outside to fill them. Bring some pregrown flowers or seeds, and have the children dig a little hole in the soil of the jug so that they can plant them. Set them in the sun and have your preschoolers keep them watered until they sprout. Take this opportunity to talk about how plants grow.
Cut a hole in the side of a milk carton (or one for each member of the class). The hole should be big enough for a bird to fit through. If the edges are sharp, sand them down a little. Have your preschooler paint the jug "nature" colors, like green and brown, or have them paint a nature scene, like flying birds. You can use any kind of paint, but if your feeder will be exposed to rain, seal it with a clear acrylic sealer. Put a little bit of loose bird seed in the center of the jug and hang it from a tree or on a shepherd's-style hook. Sit with your preschooler and watch the birds, noting how many different types you see and explaining how birds fly.
Have your preschool class work as a team to build a giant milk-jug igloo. Tell your students to bring in empty, washed milk cartons from home -- any size and any color. Let the students decorate them if you want, or have them paint the jugs all white with silvery glitter. When you have enough cartons, have some students hold the cartons while others use glue to stick them together, or have some students hold the cartons while you hot glue them together. This requires a lot of cooperation and is a great exercise in learning how to work with others.
Turn milk jugs into piggy banks by setting the milk jug on its side and imagining the cap is the snout. Cut a small slit in the top with a sharp knife for the change to enter. Glue on some stiff felt triangles for the ears, and attach a curled pipe cleaner for the tail. Let your preschooler decorate the piggy bank however she wants with paint, sequins, stickers and other craft materials. Use this opportunity to begin teaching your child about the importance of saving.