Since you are entering a position, I would talk to the company that has hired you and try to learn as much as you can about the technologies and tools used by the company. Figure out what version control system, bug tracking tools, unit test frameworks, IDEs, GUI toolkits, and so on. You won't be able to learn everything before you start, but you can definitely read about them and play around with some of them. It would help you hit the ground running at work.
Maximizing the value for both yourself and your company would be the best route here. It'll help you hit the ground running at work, and show your new company that you have some initiative and personal drive.
An alternate approach would be to learn your domain. I'm not sure what industry your company is in, but you could take a look at the current trends in that industry. Find out not only the technologies used by the company you are currently working at, but look at the leaders in the industry and see what kinds of technologies they are using. You could take a look at their software or perhaps browse through their job postings and see what they are looking for.
This would allow you mobility within your industry of choice, especially if you plan on spending a lot of your career within this industry, regardless of what company you are working for. It would also allow you to make suggestions to your company as to possible solutions to various problems that you might, if you are proficient enough with various tools and technologies that are of use in your industry.