I have tried a couple of things that have seemed to work well.
The first is an old bowling ball. You have the person hold the ball next to their hara and practice tenkan movements.
The second is plastic golf tubes. These are the tubes used in a golf bag to protect the handles and shafts of the clubs. They are very lightweight and make a nice "bonk" noise when you hit someone without causing any damage. These are great for working with a beginner for iremi practice. They are also good for paired bokken practice where you can really try to hit someone without worrying about hurting them.
Finally, one of the best tools that I have found for paired weapons is my patented "no tools at all" method. Have the students try doing the paired weapons kata without the bokken or jo and make sure they pay attention to spacing, timing, etc....just as if they were holding a real bokken or jo. For some reason this really improves timing and distance because uke and nage tend to concentrate on their partner rather than the weapon they would typically have in their hand.