I have found swinging a furibo (see big clubs at the bottom: http://www.budovideos.com/shop/custo...roductid=25376
) and doing sumo style shiko (leg lifts/stomps) are two great exercises if done right. I also enjoy doing some spear thrusts with a 7-10 foot length of black malleable 3/4" pipe. My father and girlfriend's father were both former plumbers so I have a bunch of that stuff at my finger tips. You can get it at Home Depot for $15 a length, too.
"most useful part of it?"
Do 100+ cuts with a furibo, 100+ shiko and as many tsuki with the pipe as you can 3X a week and you'll find out really quick what it is useful for
If you do it right your bokken, jo and uke will feel like feathers next time you go to class and you'll be able to cut, swing and throw with your hara, not your arms, much better.