As I suspected before even looking at the data, the services isn't as popular during the winter months which is why places like 63rd Beach see a lot less usage during those months. However, places where bikes are used as daily commute options, such as on the UIC campus, see a lot more "bang for their buck". While UIC is covered already, I think promoting to businesses a special discount rate for their company would heavily increase the number of annual members who then would choose it as their mode of daily transport to and from work.
Also, I think pointing out that people could take the bike from the subway station to their end location might be a good note for Chicagoans in the colder months. While snow will keep most people from using it during bad weather, it could convince others to use it for the shorter distances that they usually walk in order to get out of the cold faster. Or a seasonal pass that's good from April to October when most people wouldn't mind biking around the city.
As for tourists, depending on the feasability of working with other cities, perhaps an annual membership could be used back home once they leave Chicago. Otherwise, I can't see why they would choose to be an annual member.