Weightlifting/body building is fine, if that's your thing. Just be sure to be ready for what you will be doing during Plebe Summer. Be prepared to run. If you're a HS x-country runner, just keep up your workouts. If you've never been much of a runner, start now. Do lots of pushups, situps, and pullups (if possible). If you have no place to do pullups, just do more pushups.
Follow the link posted this thread (1st page?). Before arriving on I-day, do your best to be at "week 8" levels or better. Be sure to follow the advice to alternate running with swimming or biking so you don't develop shin splints.
One other thing: become properly hydrated. Most people aren't, and that will make exercise in the heat and humidity of a typical Annapolis summer a LOT harder. You may even become a "broken" Plebe.
This isn't rocket science. USNA has given you an outline of how to get ready (see link). In the words of the ad: just do it.