I just bought a pull-up bar that installs in a door frame. You can get them at any sporting goods store.
Here is what you do.
Install it in your bathroom door frame. Everytime you go to the bathroom Do a set of Pullups to failure before entering and then one after to failure when you are done using the bathroom. Do this everytime you use the bathroom. Do this everyday.
Not only will you help build muscle, you will never have to do cardio as this will burn way more calories each day, then cardio would.
Also, you can use the pull-up bar anytime you like, but make the bathroom visits mandatory.