Some member of the grass family, corn, wheat, rye, oats or like was mentioned brassica. These all will love the available N. What you need to consider is you plans down the road. By this I mean how quickly do you envision going back to clover? The reason I ask is that I like running a round-up ready crop for a year or two to deplete the weeds in the plot that tend to be the cause of the rotation in the first place. I like running corn for a few years if you have the means and ability and then overseed brassica and cereal grains into it in the late summer. Then the final fall sow your clover into it and simply let mother nature and the critters take care of the stalks and come spring you will have a great clover stand. You can do the same thing with beans, but beans don;t need the N, but again overseeding with brassica and cereal grains will appreciate the N in the soil.