Be careful when doing randori. In one hand, it will improve your "real life" aikido. In the other hand, beginners tend to forget about doing techniques the right way in order to be fast enough.
I introduced randori from suwari waza, uke was able to do just two kinds of attacks, and nage replied with kokyu or irimi nage. Then we gave more possibilities to uke 'till he was able to do any attack he wanted to do. Kokyu nage is a good start since nage doesn't need to do a lot with her/hands. Just move out of the way :^)
The advantage of doing randori in the floor is that nage generally feels less threatened and reacts more naturally.