One of the things I always wanted to try is combining watercolor and ink. This has many applications, from beautiful sketches (See Russell Stutler's page for paradigmatic examples), to illustrations. Eventually I will try to do this with fountain pens, but until those pens arrive to my mailbox and I find time for testing them, I have the possibility of using a very old friend of mine: the Staedler pen.
Staedler Mars Matic is a technical pen which I used at high school to learn technical drawing (you know, plans, designs, etc, everything with rule and paper). Technical drawings have very precise linewidth specifications which is the reason why this kind of pens are callibrated to produce consistent lines... provided you use them right! Drawing with technical pens is quite different from the fountain pen experience, as they tend to be used with an almost 90 degrees angle, more controlled motions, and in general less improvisation.
On the other hand, the ink that these beasts use is waterproof and can be combine with watercolors, as I will show below. Note, however, that the fact that this ink is waterproof also means it is hard to clean: the pens must be cleaned regularly, specially between loads. This sometimes can be done with warm water and soap, but if you forget the pens for a long time, cleaning the thinnest pens may lead to serious damages, as I learnt recently (I lost the 0.2 and 0.4mm pens a few weeks ago).
Below I show a picture of the pen as you uncap it. Notice the fine head, with a metallic tube at the end of hit. That is the tube through which the ink comes out. Inside the tube there is a fine needle attached to a metallic weight. Pressing at the tip of the tube causes the needle and the weight to move upwards, allowing the ink to flow freely but with a controlled width. Capillarity also prevents the accumulation of too much ink, but you have to move the pen slowly to get consistent lines.
The pen does not admit cartridges. Instead, it has a plastic ink deposit that has to be refilled with an eyedropper-like bottle. The ink is permanent but quite fluid, and a single load lasts for quite a long time, which is great. On the other hand there are only a couple of colors available (black and red) and I do not believe other types of inks can be used with these pens (please correct me if I am wrong).
Given all these limitations it is nevertheless fun to draw with them -- perhaps also because of some sentimental attachment that brings me back to younger times. Below I show a picture I made for my nephew, to exemplify this. The picture was sketched with pencil and then drawn over with the Staedler Mars matic. The reason is that, as I said before, these pens are not good for sketching but excel on producing consistent lines, which is easy once you have a pencil sketch. On top of this I applied watercolor (sorry, nothing fancy, I am not an expert), and then drew again to get mor eintense blacks and emphasize the original contours and shadows. Overall, a lot of fun!