I only do art once a week and take out the crayons/markers on a limited basis so that it is something special to do. If you are finding they aren't playing with anything then it is probably because they have too much. Try putting half the stuff away and see if it changes. It can take awhile for them to get into playing on their own if they are used to always having someone lead the play. Imaginative play doesn't really happen until well into their 3rd year I have found, and actually playing with other kids (as opposed to near them) doesn't happen until around 3 years old either.
I do similar to what Crayola kiddies does with her toys. Some are always available and others are rotated.