AOPA eFIRC online is by far the most interesting refresher and easy to complete in a day or two. They also have the easiest renewal processing without having to do all the faxing and mailing of stuff, it's all online. I've done Gleim several times, and only heard how boring and outdated American Fliers is to use. If you're not trying to go the cheap route and actually want to get something out of it and have them literally handle everything for you, try AOPA. Flight Instructor Refresher Courses - AOPA