Do you like running ? How about doing a Couch to 5K program ? That can be done on a treadmill if you don't want to run outside. www.sparkpeople.com/resource/fitness_artic
An intermediate program is kinda relative. What might be intermediate for one person may be too advanced or even easy for another.
What sorts of activities do YOU like doing ? Do you like running ? cycling ? Do you like classes ? If you feel you are active, but want to be more fit, how about trying some of the classes that are offered by your gym. Try Zumba. Try spinning. Try kickboxing. there are lots of different things you could do in 12 weeks. Some gyms even have 8 week boot camp classes. That's something you could look into as well.
Of course, there is also Coach Nicole's 28 Boot Camp DVD workout. www.sparkpeople.com/myspark/28daybootcamp.
Here's the thing, what you do for exercise depends on YOUR goals. What goals do you hope to achieve in that 12 week period ? Once you've decided what those specific goals are, then you can look into different ways to achieve those goals.