Oooh, I run an eco-crafts club with my students and if you did some prep before hand you could do some neat projects.
When is the party?
If you can find #6 recyclable plastic (usually like the boxes take out food comes in or cakes from supermarkets) they make EXCELLENT shrinkie-dink material, so they or to be safer you, can cut out (fairly large -- they shrink like CRAZY) funky shapes and let them decorate them with permanent magic markers (Sharpies have the best colors I think) and make sure you punch a hole in the top and then shrink them in the oven (very low for about three to five minutes or so) and then buy some jewellry making stuff like earring hooks or necklace cords and clasps, or bracelet strings and let them have a piece of take home jewellery...here's a tutorial: http://www.instructables.com/id/Dood...Recycling-6-p/
Or doing flower mobiles from the bottoms and tops of soda bottles...you just cut them in flower shapes and paint the insides in fabulous colors like flowers, then poke tiny wee holes through the sides and string them on fishing wire and hang them in mobiles...so cute!http://www.easy-child-crafts.com/pla...le-crafts.html
Actually this site has LOADS of eco-crafty ideas: http://www.artistshelpingchildren.or...ideaskids.html
and this onehttp://www.easy-child-crafts.com/index.html
For other favors if you want you could give out those mini tubs of playdoh or maybe those mini palettes of watercolors like you could find at the dollar store.
Would it be crazy to have a build your own mini pizza for the food?