It's pretty damned difficult for women to build serious muscle. Lots of women fear getting bulky but it takes some serious work to get to that stage, like years and years of regular heavy lifting and eating a lot of protein.
Lift weights; you won't get bulky and the muscle you do build will burn more calories at rest than fat. If you want to boost your metabolism, lift weights and build more muscle. Also, for what it's worth, having a little muscle on you won't lose you your feminine build, it might even enhance it. Barbell hip thrusts are magical.