Leg raises are an excellent exercise that engages your abdominal muscles while building strength in that region. It's also excellent for lower abs.
The lying Russian twists are an excellent exercise that builds your core and abs while chiseling that coveted "V" of the lower abs.
These exercises target the calf muscles and promote hypertrophy without stressing the upper legs and growth in that region while giving them the chiseled look.
Leg presses are an excellent way to engage the legs without stressing out the back or any other regions of the body.
Dumbbells are free weights that allow for a more concentrated movement and hence are part of most strength training programs when it comes to building arm strength.