Have a Garden Party!
Make a mini seed-starting tray. Recycle a plastic egg carton -- the see through ones make awesome seed starters -- take off the rectangular top, put it on the bottom as a tray, poke small holes in the egg bottom, fill with soil, and voila! a mini-greenhouse! last year’s seeds are sometimes 4 packets/ $1 at our local garden center, and will still have a really effective sprout rate.
Do the setup for a grass head guy. You get an old stocking or ped from shoestore, pour a layer of grass seeds in, top with lots of pot soil, knot and plop (knot side down, grass seeds up) into a pot (paint the pot first like a face or cool pattern). Tell them to keep it moist on a sunny windowsill and voila, monsieur le grass head. You could buy little terrra cotta pots to paint the face on, or look around for something more interesting. Go to the thrift store and see what mugs, ice cream glasses, tea cups etc they have for cheap. I've also seen this idea done with a sponge. Cut the sponge into a shape (star, for example), moisten, cover with seeds and put in a shallow plate on a sunny windowsill, keeping moist.
For the Birds
Recycle 1/2 gallon milk or juice cartons and soften a nest while you're at it!. Cut a square "window" in two opposite sides of the carton (the sides perpindicular to the table, not the slanted top sides). Use a hole punch to make an opening about 1 inch below the cut out portion in the center of the carton. insert a twig through the holes for birds to stand on (it should extend a few inches beyond both sides of the box). Paint it if you like in bright colors to attract attention. Fill with small pieces of yarn, dust bunnies, pet hair, people hair, dryer lint, etc. Hang from a tree or near a window so you can watch the birds gathering fluff for their nest.
Also, this winter as gifts for friends, we got cool jello molds, and mixed birdfood with corn syrup. packed into oiled molds, baked for 5-10 minutes and we had shapes. They melted at the first sign of rain (and many of them broke before that -- I wonder if molasses would work better. do birds like molasses? I would think peanut butter would be too messy.... shortening? mixing just enough to bind?) anyway, we put a nail through the center, threaded yarn through along wtih the nail and hung them on our shepherd's hook. it was a fun, tactile project the kids (and the squirrels who scored all the seed that fell) enjoyed.
Spring Egg Fun
Make a papier mache egg using small water balloons, flour and water paste (add salt to discourage mold) and newspaper strips. have some pre-ripped, but my kids love the satisfying feel of ripping paper. condoned destruction! nothing like it. cover the eggs with newspaper dipped in the paste. satisfyingly messy, but relatively easy to wipe up. 'specially if you use a disposable table covering (sheets of newspaper?)
Or you could do the ones with thread and liquid starch. Give each child a small bowl or ramekin with some laundry starch and a spool of thread. pull out some lead thread, put it all in the bowl. Blow up balloon, wrap starchy thread aroudn balloon til happy. Let dry, pop balloon, and you have a cool lookin egg.
You could also do this with a bigger balloon depending on ages of kids, and then once dry, they could cut out a round doorway in the side, stuff it to just below the doorway with that atrocious plastic "grass" ubiquitous at this time of year, or use a natural alternative like raffia, sphagum moss, etc and make some kind of creature out of playdough or scupley or that DAS clay that air dries to live in this nest.
there are literally tons of craft ideas out there that you can do for pretty cheap, and using the thrift store or recycle bin you can make some of them even cheaper. And it's so much better than that evil-smelling plastic "favors" from oriental trading, etc.
just my 2 cents to get you started. hope that helps!