One thing we always did with COR events is hand deliver the invitations. We would have our PR committee make up personalized invitiations for all the PNMs and then we would delegate a sister who knew the PNM to deliver the invitation to her in her room. That way if the PNM had any questions she could just ask the sister and not be nervous about calling the recruitment chair which can be daunting to some girls. Plus it showed the girls that we were interested in having them there and didnt just go to the post office and mail out a mass number of generic invitations. Invite the girl to bring her other friends if she feels intimidated by going alone if she doesnt know anyone else, that makes it a win-win situation for everyone.
The more one on one COR events that we had always seemed to work best. We pretty much had a BBQ event and a smores making event every fall. We had the best turn out for those because there was food and who doesnt like chocolate?
But I agree with GreekLetterGirl the theme doesnt matter much, it just gives you a conversation topic to fall back on.