Don't forget that children learn through repetition - from babyhood through the early childhood years they will do some things over and over again, from shaking and dropping toys as infants, to dumping out all the toys as toddlers, to building block towers and crashing them down before building them back again in the preschool room.
To some degree, the endless stream of crayoned self portraits and sunshine and rainbows drawings you see at the art center are a continuation of this style of learning! As such, you probably don't want to let the children know that you are getting bored with their drawings. :)
With that said, there are some things you could do to "jazz up" their coloring/drawing experiences:
-Try decorating cardboard boxes or other 3-D items. Cardboard tubes can be fun to decorate.
-Tape paper under the table and let kids lie on their backs to color "upside down".
-Use masking tape to tape bundles of crayons together, allowing children to draw with several colors at once.
-Draw on long strips of paper adding machine tape. Fun by itself, or introduce some adhesive tape and watch the kids create bracelets, belts, headbands, and more!
-Punch a hole in a paper plate and place on a record player; kids can draw on the plate as it spins round and round!
-Decorated paper plates can also be cut is a spiral to make "snakes". Kind of crafty, but the kids love it!
-If you can find an old low-temperature warming tray, these are AMAZING with crayons. They get warm enough to melt the crayons, without getting hot enough to burn the children.
-Try drawing with white crayons on white paper, then brushing over the entire page with watercolor paints (or a cotton ball dipped in a little tinted water) to reveal your "invisible" pictures.
-When you're feeling especially silly, try taking off shoes and socks and drawing with your feet!