I am a former college basketball player and current basketball skill/strength coach. I have a ton of free drills on my basketball training website, The World of Hoops(just click on basketball training link to get to the site). In addition to that we are about to release the ultimate 12 week off-season training book. It will be up very soon and you can get it through a download and google checkout. Let me know if you want me to let you know when it is available. In the meantime, check out the site for tons of free drils - shooting, defense, rebounding, ball handling, passing, and sttrength training.
Good luck, hit me up if you have questions.
The best thing that you could do in the offseason is to practice, practice, and practice some more. Sometimes, i like to run around the block every other morning. However, practice wins games.