Drop Dead Legs
I've always found Aikido a vey leggy activity.
It's all the getting up from falling down. Even after a laid back class I feel it in my thighs and quads, so any exercise that increases upper leg strength may be of benefit. I'm not saying it'll make you a better Aikidoka, but it may make your life a little easier.
Also, I find working out on a punch
bag, although not very Aikido, I know, gives me the same "burn" as a randori session.
Hope this is usefull.