Maggiebee, go for non-weight bearing exercise such as swimming as woofgang has suggested.
Anything that is a non impact type activity but requires you to use your muscles will work as well, for example cycling or yoga - but I'd start with swimming first just to ensure that you are confident with the range of movement you now have and also the amount of exertion you can put on the joint without it starting to ache.
You could use a small float so that you're arms don't tire so quickly, allowing you to do more lengths per session which will help increase the muscle around the joint on a steadier rate than without, and a float will allow you to breath easier when doing the front crawl leg stroke.
Good Luck
Or, I could be completely off the mark here and you could be super-fit already - in which case, to increase leg strength try squats or lunges and if you are a gym-goer, use the Smith Machine with the straps on your ankles to strengthen around the hip joint.
Have a quick peek at this link - any 'Adduction and Abduction' leg exercises will help to tone/ gradually increase muscle strength around the hip joint - some of the videos at the bottom will show you how to do the exercises at home so you don't have to break the bank by joining a gym.
All the best x