Lower leg, knee or upper leg?
You can do push ups (in your knees if you aren't used to doing the military type or even "wall" push ups
http://www.washingtonpost.com/wp-dyn/content/video/2005/08/01/VI2005080100611.html) for cardio and alternating with crunches
http://web.utk.edu/~dwalke13/weightlifting/crunches.html as an alternative to the elliptical until you get better from your leg problem. I don't know on what level of fitness you are but you could do what I call "Pyramids". You start with 10 push ups and 10 crunches with no rest in between until a count on one. 10 push ups and 10 crunches, then 9 push ups and 9 crunches, then 8 push up and 8 crunches and so on ...
Also, you could get one of those exercise balls. Believe it or not, you have a great range of exercises you can do with those. And they aren't expensive if you consider all the exercises you can do with them.
http://www.resistaball.com/articles/RAB_insert.pdf