I agree with mixing it up, and particularly with adding circuit weight workouts (that move quickly from one exercise to another without resting), or workouts that combine cardio and weights.
Building muscle takes effort, and an increase in calories. If you keep your eating under control, and don't add calories, you shouldn't build a lot of muscle (just tone what you have) but the weight work will help increase your metabolism.
If you start to get a little bigger temporarily (because of being new to weights, and fluid retention in the muscle as it gets used to the new work), don't panic! You're building some shape, and not just making what you have smaller, so the end results will be nicer.