Twist and shout
I find that the warm up exercises and stretching at the beginning of class are not enough. They don't really do much good, because they're over way too quickly. In fact, it's pretty darn dangerous, considering the extent of ukemi and general twisting and turning that we go through -- ouch!!!
I know that it's different for each school, but of the classes that I've been in (and watched), they're not done enough to do much good (I even had one sensei tell me that students come to him to learn techniques, not to get a workout. I couldn't take my eyes off his distended gut as he explained this to me).
I think warm-ups/stretching should last at least fifteen-twenty minutes -- minimum.
And why aren't pushups or situps, etc. done? They just increase strength, general health, etc., it doesn't matter if strength isn't used in aikido.