Interval training is an excellent way to kick up your metabolism. Try a walk for a couple of minutes, then a jog for a few minutes, then back to a walk, then up to a sprint (if you can) for a minute or two etc etc. It really does make a big impact. Also another good way to increase your metabolism is weight training, not sure if your gym does group fitness but if it does try a Pump class. It's fun, a fab workout and 2 or 3 a week will see you notice a big difference in your tone in 6 weeks.
Good luck.