Couple of thoughts. First, if the audience is nominal American Protestants, it may be prudent not to assume that these folks are Christian, except in a superficial sense. I would start with the basics of the Gospel and use the Holy Scriptures, which are sufficient for this initial stage.
Second, it seems to me that the "how" of catechisis is just important as the "what." You might want to check out a structured process that is perhaps exemplified by the "Putting on Christ" program at the Sint John the Forerunner Orthodox Church (Antiochian Archdiocese) of Cedar Park, Texas. http://www.theforerunner.org/pubdocs/PuttingOnChrist.pdf
. From this brochure, here are the tasks that each catechumen must accomplish:
"We expect catechumens to attend at least 20 sessions of Orthodox Instruction on Saturday afternoons.
We expect catechumens to attend the services for 8 out of the 12 great feasts during the liturgical year.
We expect catechumens to read four books about the Orthodox Church or the Orthodox Christian life.
We expect catechumens to participate in the services of the Church on a regular basis.
We expect catechumens to contribute to the life of the parish through gifts of time, talents, and money.
These tasks can be modified to fit a person’s particular situation: for example, due to work or a family commitment, some people cannot attend very many classes on Saturday afternoons; so these folks read additional books. These sorts of modifications are very common, but the tasks themselves are an absolute requirement."
It helps that the rector of St John, Father Aidan Wilcoxson, is aided by a Director of Catechumens, Sub-deacon Thomas Wilson. (Incidentally, on another thread somebody asked about the utility of sub-deacons- here is a stellar example)