A great site for craft & art ideas is Crayola.

My kids have gotten great ideas from this site before.

Also, Oriental Trading is a really inexpensive catalog store for crafts. These aren't generally as conducive to total creativity - largely they consist of pre-packaged crafts. But it's nice for a big group, since the crafts are individually packaged.

If your artistic children are interested in an art contest, there is the Youth for Human Rights International What Human Rights Means to You annual art contest.

I've seen some of the artwork created by contest participants in past years - really good stuff!