Now, physical training is certainly the engine that drives intensity in standard Aikido training, but there are certainly other ways of creating intensity, at least in theory.
Chiba Sensei said he uses the zafu toward this end. His reasoning was that the mat could never get intense enough, after a while, because it must always remain a partially controlled environment. In my experience, I not only feel the mat cannot get intense enough but that the cushion cannot either - for the same reason. For this reason, I try and look for two other areas of application/cultivation: law enforcement applications (which can get very intense, but not that often) and everyday relationship with those around me (which is by far the most intense training areas around). Don't get me wrong, it's always going to be easy to practice various forms of ignorance, alienation, and apathy, with our spouses, our children, our aging parents, our co-workers, our friends, and those that are strangers to us. But, if you commitment yourself to a broader application of Aiki, where wisdom, intimacy, and openness have to be present in everything you do, say, and think, then your day-to-day encounters with others are going to make the mat, any mat, feel like a break from it all.