Make an effort to get some extra walking in. If you go to the mall, walk an extra lap around just for the exercise. If you take the bus, start using the one stop farther away from your house. On your lunch break, go for a short walk before going back to work.
I know extra few minutes isn't "exercise" but I'm of the opinion every little bit helps.
BMI is completely and absolutely false. It doesn't take into account too many things and the majority of the time is very inaccurate. But you are a perfect weight darling.
Weight is more about eating healthily than exercise.