I decided to put the innards of my Wacom in an erasable "magic slate" to fix that problem and make the overall experience more touchable and fun. I wired it so that sliding the writing surface of the slate out clears both the slate's screen and the computer drawing program's screen.Then I converted the tablet's outer casing into a watercolor palette.
For the Magic Tablet
1 Wacom or other digital graphics tablet (eBay-ed or old is good, there's a chance of damaging it)
1 magic slate (this one for instance: http://www.amazon.com/Magic-Drawing-Slate-by-Schylling/dp/B000ICZ5IW/ref=sr_1_1?ie=UTF8&s=toys-and-games&qid=1226218291&sr=1-1)
Conductive fabric, copper tape, or aluminum foil
Some wire, solder, and a soldering station
Glue, hot glue, scissors, possibly a utility knife (Wikipedia says this is the proper generic name for an x-acto :)
Bits of cardstock
For the "Digital Watercolor Set"
Outside casing for a Wacom "Bamboo Fun"
Watercolors in tubes
A hot glue gun
A plastic bottled water cap
Step 1: Disassemble Tablet
Very carefully pry the casing apart (using a flat screwdriver to gradually pry up the edges helps). If you're using the tablet I did (the Wacom "Bamboo Fun") you can pull out the scroll touchpad from a little plastic slot but BE CAREFUL! I damaged the connections when pulling mine out and ended up breaking the touchpad. Carefully remove the board and the insulating metallic sheet behind it.
I really ought to have more pictures for this step, but I disassembled mine a long time ago. It's pretty straightforward, but let me know if I can offer any assistance.
Step 2: Disassemble Magic Slate
Interesting note here: when I took mine apart I noticed that the erasing cardboard tab that helps separate the wax sheet and the plastic on top was cut out from a different children's toy or cartoon page. It's not visible unless you take it apart so it didn't matter if it matched the outside design. I thought this was a very cool use of recycling, wherever these are assembled.
As an aside, I'd always wondered how those magic slates work. Basically the bottom layer is coated in dark wax. When you press down on the top plastic layer it sticks to the wax, making it show through in that spot. When you separate the layers the image clears.
Step 3: Solder Connections (optional)
You can certainly skip this step, it does involve a little more risk to the tablet.
I made the conductive patches out of conductive fabric (lessemf.com) because I happened to have some around and because it seemed the most likely to make a flat, smooth surface over which the two pieces could slide. Aluminum foil or copper tape would probably work well too.
First, cut out two small pieces of conductive fabric and solder a wire to each one. Move quickly, the fabric is relatively easy to solder to but it does burn through fast. Glue each of these patches, solder side down, to the cardboard frame and the writing surface (see pictures for location).
Next, use pliers to pry a button out of its little socket. Solder the ends of these two pieces of wire to each side to create a switch.
The wire that goes to the conductive patch on the backing should just be long enough to reach the button on the tablet (red wire in photo). The one on the drawing surface should be long enough to allow it to slide freely (black wire in photo).
Step 4: Add Wacom
Step 5: Reassemble
Cut a small hole in the backing at the level of the USB connector.
Cover the tablet with a piece of cardstock and glue it down. This will protect it a bit, since the drawing surface will slide over it frequently.
Since my tablet was pretty small, the writable magic slate area was larger than the writable tablet area. You can increase the width of the frame (bezel) by adding strips of paper to the opposite side of the clear plastic window.
Glue the backing back on the Magic Slate, enclosing the tablet inside.
Step 6: Set Up Software
Plug the tablet in and test it again. You may have to mess with the switch a bit to make sure it is activated when you pull the drawing surface out.
Step 7: Make Palette From the Rest
Then, squeeze blobs of watercolor onto the surface of the case.
Step 8: Improve!
I'd like to come up with a more robust switch mechanism.
I'd like to add more buttons that look like tools (select tool, color palette, etc.). They could be hooked up to an Arduino board and processed by a drawing application.
I'd like to write the aforementioned drawing application specifically for this interface (it looks like a drawing tablet and can read the additional buttons).