Boxing is cardio, so is cycling. They'll both help you lose weight! The reason boxers are beefcakes (mmmm boxers...) Is because mostly they lift weights too, to help with the strength. The main technique, "float like a butterfly, sting like a bee..." is cardio. Try keeping on your tiptoes and moving around the bag more. Treat your bag as an opponent and hit it when it's swinging towards you. You can also try varying the power of your punches on the bag.
I really like a "no contact" spar with my fella, so you can practise technique and have a real life opponent to react to!
If you want to build muscle (which in turn burns fat and also takes up less space than the same weight of fat) then try starting with some dumbells and
www.stumptuous.com/iron. You can also head over to LWL thread for more fabulous advice!
Oh and as for the combination, anything you enjoy is GREAT exercise! Don't stop moving!