What do i need to do to tone/tighten /thighs area w/o losing weight?

I am 5'5 and weigh 108 lbs so for my height I'm underweight but I feel really good just how I am and have no interest in gaining or losing any weight! However, I would like to have a firmer , and to tone and tighten my thighs! So what kind of exercises regimen do I need to have?? I'm unsure of wether I should do cardio or how often because I don't want to lose any weight. Or if I should just do lower strength exercises and how often??? HELP PLEASEE