Chytrak

Aikido is about 99% defensive in nature. I train with a guy in Tae Soo Do who took Aikido for a few years and he said there was really no sparring involved because there are no offensive techniques in Aikido. I haven't taken Aikido, so I cannot attest to the amount of physical activity, although I am sure it will not be like a high school gym class.
Currently, I take Tae Soo Do/Hwa Rang Do and it definately has the disipline and physical activity you mentioned. Just about any martial art that you study will offer disipline and physical activity. Best thing to do is to research and then try out a class or two at a school in your area. One of the things to look for when trying a class is how many instructors they have on the floor during class. When a person is trying out for the first time in one of the Tae Soo Do classes, the head instructor will usually appoint one of his assistants (who are just as knowledgable and capable) to train with the prospective student exclusively for the entire class.