Lunge and twist - with a dumbell held horizontally at chest height, do a lunge, when you're all the way down (don't touch your knee), twist 90* towards your forward leg. Stand back up. Repeat the other leg forward. Can do them "walking" or stationary. Good core/lower back/legs.
Full squats with dumbells - With a dumbell held by the bell end (hanging basically) at chest height, do a squat ALL THE WAY DOWN. Don't just go parallel, go to a catchers position, then back up. Good for quads and lower back (keep your back straight, and it will burn by your third set).
Good mornings - dumbell held at waist level, horizontally, bend at the waist with straight legs...hold it at the bottom to stretch the hamstrings. Use nothing but lower back muscles to return upright. This one will destroy your back for a week if you're not used to it.
I work out with light weights compared to most, but I do high-intensity cardio type workouts...burns fat better and keeps muscle tone. I do 20 lbs for lung-twists, 30 lbs for the deep squats, and 40 lbs for the good mornings. I do 1 minute sets, as many reps as can be done, 15 second pause between sets.