I am far from an expert, my choice would depend a lot on the size of the plot and deer density. Soybeans are actually very drought tolerant, but as previously mentioned, they can't handle a lot of pressure. One option, if the beans do fail, would be planting a rye/clover mix in September. With a decent amount of rain, you could still have a good plot for fall.
If you are able to maintain it, alfalfa is a good option.
4 point, how big of a sunflower plot do you put in? Do the deer hit them summer through fall?