Kokyu Ho breathing; Sitting in Seiza. Shikko Ho. General exercise.
Yeah, I would definitely second these, although with the caveat that if you are a real newbie at aikido, by practicing any basic aikido movements like shikko ho without oversight, you may be training yourself into bad habits.
There was a discussion at one point on this forum about good exercises for aikido. I have given the matter a lot of thought over the last 6 months and cannot say I have any really good answer. I have been trying different things in the gym while continuing to train with this year's Kenshusei class at Mugenjuku. Every exercise besides simply doing aikido seems to have a down side as well as a benefit.
One conclusion I have come to is that if you are going to work out for aikido, how
you do exercises is very important. For example, I think the Roman chair leg raise
is likely to be one of the best exercises for aikido; however, it is true only if
you do it in certain ways. For example, you need to not rest your back against something as in the link I posted. Instead, focus on keeping your shoulders from rolling forward and then use your lats and the muscles around your spine to hold this shoulders-back posture with your chest and spine expanded. In this position, you don't need to rest your back on something and you can experiment with manipulating your hanging pelvic region while keeping good posture for aikido.
My guess is that a properly executed Olympic Full Snatch is also an excellent exercise for aikido, although I haven't tried incorporating it into my workouts because my knees can't take it.
I don't think running is particularly good. If you are going to run, make sure you keep your legs stretched out, especially the gluts, ham strings and shins. Tight gluts and hams seem to be big contributors to poor seiza and shikko ho.
I don't think traditional crunches are very good for aikido, maybe even counterproductive. I think exercises where you are anatomically extended (like Back Extension) and your lungs are open are better than exercises where you hunch your shoulders and curl up.
I would guess yoga is good, although I haven't tried it because I don't like it.
If you are stuck at home, you can do air squats and you could do Full Snatches at home, too, with a jo, a bo, or a broom handle. The point is to use the bar to control your posture.
Well, that is all for now. Good luck.