Not quite what you're looking for, but you can take a look at
ObjectGraph (objectgraph.rubyforge.org) at how to dymanically evaluate
classes and generate a png graph using GraphViz. You can set some
graphviz options to make the output *look* a bit more UMLish, but it's
not proper UML.

I am not aware of any tool that reads ruby to generate UML (and would
love to hear if there are!). You might have to follow the same process
as ObjectGraph, but generating an XMI document to load into an existing
tool.

There is just one more question that need to be asked: are you sure
that is what you need? I find that because of ruby compactness of code
and duck-typing arguments you rarely use much of inheritance etc. As
for activity diagrams, you can not normally get them from static code
analysis so you'd have to do them by hand anyway.

Cheers,
Assaph