Hi Thomas,
I don't really know off the top of my head any specific designs to recommend for you to work on. As you may have noticed the ones I did were mostly clustered around a small group of designers, so one idea is to look at other designs by these same designers. I do recall there being a few designs by ramblingsoul that I didn't make skins with but might have if I had more time to throw at it. Often you may find that if you pick one of those designers, you can use one of the existing skins by that designer as a starting point. For example when I did the dcarter skins, after the first one I always used one of the previous ones to start the next one and there were structural similarities that made it go faster.
If you really want to get into web design and making skins, I would recommend getting a few good books on CSS and reading them. The second and third book from the top on the right side of the Creating Skins page are the ones I would start with. After reading those I think skining will start to become easier for you and you will even be able to do complex things like menus, though its still a somewhat time consuming trial and error process. But its also fun and rewarding.
Best,
Joe