I think it's easiest to come together spur-of-the-moment and do a fun project with your kids if you have some basic supplies always on hand. Here are some of the things I like to have on hand. (They're all pretty inexpensive, especially if you can take advantage of sales!)
* CRAYONS ~ I totally stocked up on dozens of boxes at a quarter a piece in a Back-to-school sale! Coloring is always more fun with a fresh box of sharp crayons and you can also do Crayon Painting or melt them to make your own crayon shapes.
* GLUE & GLUE STICKS ~ You can never have too many of these either. They can be used for their intended adhesive purpose or even in a recipe. Sadly, my kids never seem to put the lids back on their glue sticks, so they tend to be one-time use items :( I don't get too upset, though, if I stocked up when they were just pennies a piece.)
* FOAM BRUSHES ~ Stock up when the craft stores run deals on these! They're great for sidewalk paint and to easily paint large areas.
* CORNSTARCH ~ Sounds odd, but you can use it for so much more than just making gravy! Sidewalk Paint and Magic Mud are all super fun and cheap activities using cornstarch as one of the main ingredients. It's right around a dollar a box and you can keep it in your pantry forever.
* FOOD COLORING ~ It's a couple bucks more than some of the other items on this supply list, but you can use it for everything from playdoh to paint.
* PIPE CLEANERS ~ Great for just bending into shapes as well as lots of other little projects.
* ACRYLIC PAINTS ~ Cons: They're pretty much permanent if they get on clothes, upholstery or other fabrics. Pros: They come in lots of colors and won't rub off or smear once dry. I'd recommend letting your child pick out some of their favorite colors the next time they're on sale at the craft store and just make sure you have a smock or oversized shirt when you start painting!
* BORAX ~ It's a powder found in the laundry aisle. Buy a box the next time you go to the store, then use it to make Gak with your kids. It'll last you a long time and I promise it'll be worth it!
I know I'll think of more great supplies to have on hand as I share other ideas. What are some of your most-used items?