One is a sign generator. The SDL library lets you use Truetype fonts that you bundle with your app to draw to the screen. Take some interesting fonts, add some glow effects, and you can make a nice app for people to make messages that can be seen across a room. A "party sign" app was one of the big winners in the original Hot Apps promotion, so it would be natural to one-up that with the PDK.
Another idea is a nice 3D world time app. Show a nice 3D globe based on the current time with accurate sun charts, maybe allow touches on the screen to rotate the globe with a button to recenter it on the current GPS location.
A third idea is make a OpenGL ES-based photo viewer with some nice zoom and pan effects, aka Ken Burns mode. You can build your own copy of libcurl to do HTTP fetching of images, and grab from different Flickr tagged feeds. The UI might be a little tricky, but you can do a lot with just specializing in your own topics and maybe allowing the user to customize by editing a file read from /media/internal.
Finally, there's plenty of fun to be had by porting some of the existing SDL demos and games that are listed at http://www.libsdl.org. We fully support posting open source software in the catalog, just be sure to use a open source developer account and follow the license. That might mean adding information to your catalog description on how to get the source and build it yourself and adding notice to the user about the license terms.