First, I would ask your daughter what she is most interested in doing and what she would enjoy the most. Certain activities -- dance, gymnastics, and sports that advance to "club" level -- take on a more serious emphasis and time-consuming level if a child continues them into her adolescent years. If you daughter wants to continue participating in such activities, she should really enjoy it and be prepared to cut back on other activities because there are only so many hours in the day! While the adolescent years are a great time to sample various school/sports clubs, to see what "fits best," remind her to investigate an activity fully before jumping in with a gifted child's typical enthusiasm. It's tricky to get the sporting goods store to take back expensive, now-used equipment when kids decide that the sport is just not for them.
It's important to help the gifted student find balance in her life. At the middle-school level, there is likely to be one to one-and-a-half hours of homework per night for a student in all advanced classes. (A well-used study hall can also count as homework time). In high school, this can increase to two to three hours per night, depending on the course load. It is important that the gifted student learn to schedule in study time just like she does other activities. By the way, remind her to allow time for study while she is still awake! For many older teens, study occurs between 11 p.m. and 1 a.m. Once study time is secured on the calendar, I recommend that gifted students have one musical activity and one physical activity on their schedule in any one season. Here is where the student will try to use her gifted math skills on her parents to convince them that this is not enough! A rigorous dance program (more than once a week) counts as the physical activity or sport. The school band (especially high-school band or orchestra) counts as the musical activity. Your child could have a weekly private instrumental music lesson as well, if needed. Extra lessons in voice or drama, or starring in the local musical comedy production count as other activities and may be too much for your child. You know your own child and must be the final decision-maker. A service activity is also a good idea. This could be a school club such as a Junior Kiwanis program, Junior Achievement, or scouting.
Down the road, colleges will look for the gifted student with a balanced program of strong academics and service activities. A student cannot qualify for entry in National Honor Society without service activities. I realize that the middle-school years seem a bit early to be thinking of college applications, but you are helping your daughter set a pattern of extracurriculars that will serve her best over the coming years.
Encourage her to participate in occasional academic competitions (writing competitions, quiz bowls, spelling or history bees, etc.). These are generally not too time-consuming at the middle-school level.
Finally the most important thing for parents to remember when considering outside activities for their child is this: You have your own life, too. If you have to be away from home every night of the week because of your children's activities, they are doing too much (and so are you). If all your family dinners are becoming fast food take-outs, they are doing too much. Too often, I have seen parents of gifted kids let their own lives and relationships suffer because of over-scheduled kids. I have even had couples in my counseling practice who are on the brink of divorce because they never spend time together due to too many kids' scheduled activities. Never forget that you are the best judge of how much your child should do -- with time left over for them to just be a kid!