Seashore is a nice free editor based around GIMP, but simpler.
Otherwise, you You could usetry Paintbrush, which looks like Paint (free). As quoted from its website :
Paintbrush is a simple paint program for Mac OS X, reminiscent of Microsoft Paint and Apple’s own now-defunct MacPaint. It provides users with a means to make simple images quickly, something which has been noticeably absent from the Mac for years.