Find open tickets and submit patches to them. Rails has grown immensely from people finding optimizations / bugs / feature improvements and submitting patches. The only list per-say of what can and needs to be done is at dev, no single person keeps track of everything that can / could be done.
First of all, do not let me dampen your enthusiasm at all.
Now, I'd much rather have tickets and patches done by someone who's directly interested in that area of Rails. If you haven't used Rails enough to have some opinion on this yourself, then jump into the existing tickets and find one that interests you. Add tests. If the documentation is sketchy, clean it up with a documentation patch (which is just RDoc anyway).
There's no sense seeking out places to tweak when there are so many tickets already that others have taken the time to create. For some, they may not have the skill to create the failing test cases that demonstrate the desired change to the system. If you can give a patch for the new tests separate from the patch that fixes the code, even better!
Rob Biedenharn http://agileconsultingllc.com