Paint is always fun. You can have the boys use different methods: watercolor is beautiful, regular finger paints (yes, even at age 10!), put paint in spray/squirt bottles, dot paints (similar to bingo inkers), sponge paint, etc.
Side walk chalk is entertaining. You can take a quick picture of their artwork and have the boys make popsicle stick frames for the picture.
So, popsicle stick boxes with lids, houses, picture frames, puppets, etc.
Collages are cool. Maybe they can make a themed book collage where they cut and paste pictures that are all related together.
Dioramas are super cool and the boys can make several to play with. They can use modeling clay, painted with watercolors, to create characters. You can check out some instructions at http://vathena.arc.nasa.gov/curric/land/wetland/diorama.html
"Found" art, or nature art, is where you have the boys gather materials found around the yard and then glue them together to create another image. They can even do this with seashells to make mini animals.
Another real cool and challenging project is pipe cleaner animals. You can get step by step instructions at http://www.marthastewart.com/portal/site/mslo/menuitem.3a...
And lastly, origami! You can purchase a kit at Michaels (take advantage of their weekly 40% off coupon in your local paper) or check out http://www.origami-instructions.com/simple-origami.html
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