Also, remember that they're very used to thinking in procedural terms.
Don't forget that.

Walk them through refactoring a big procedural problem they may be
familiar with to a more Ruby/OO solution, and try to explain a bit
about the WHY of doing it the OO way.

Most of them should appreciate how less hypergolic Ruby is compared to COBOL...

If they're mainframe types, there will be some mainframe/COBOL idioms
that they may be very comfortable with that they may throw at you in
resistance. If you can counter some of them, if only to say, "it
doesn't work that way here, but this does the same thing", that could
be good, too.