I saw this video on the Kaze Uta Budo Kai forum, and found a few interesting things to play with.
1) Around the 0:30 mark, on of the gentlemen does a few back-falls while taking a step backward or forward with one foot. Normally, we practice back-falls from a static position (feet together). Once in a while, I think it's a good thing to introduce a little motion to the action once a student has the basic motion down.
2) Now, that much, I'd seen before. The part that I had not seen was practicing the back-fall with a partner, as they do at the 0:54 mark. We tried it in aikido this morning, but from a slightly different set-up. We started with both arms up, our palms meeting, with slight pressure. Tori begins to walk backwards while uke follows him (moving forward), trying to maintain a constant pace and pressure. At some point uke changes the equation slightly by stepping a little further/deeper than normal. Not much, just enough change to tip tori over his heels.
3) The we did something similar to what they do at the 3:57 mark for forward rolls. When we tried it from static, as they do in the video, we found we didn't quite get an honest "oh, crap!" sort of off-balance; tori would pitch forward, but he could stop himself without having to roll. So we tried it walking, arms up just as in the previous exercise, and at some point while moving back, uke stepped off the line to one side or the other. Then, tori's forward moment was enough to force him to roll.
Just a few fun things to try to break up the monotony.