Road biking workouts?

So I used to be runner but I now have tendentious in both my knees, and now I have a spranged ankle. I bought a new road bike and I have taken it out quite a bit. Im in college so I dont always have time to go out and bike 30 miles everyday. I just want to know if there is other workouts I can do on my bike besides biking for miles on end?

We just ride for miles on end :) And a hearty welcome to TE.

Speed, distance, hills, off road .... in your area there should be lots of challenges.