The purpose of cardio is to basically burn calories. It really doesn't matter whether you run, bike, swim, etc.
Do whatever you enjoy and constantly mix it up so you don't get bored. If you live in a city where it's fairly warm outside, try going for a run outside, going for a hike, things like that.
I live in Canada where it's winter 8 months of the year. I pretty much always have to do my cardio in the gym because there's usually snow in the ground. Try using the step mill. It is by far the hardest piece of cardio equipment. Use the machine where the stairs actually revolve so there is no way you can cheap.
30 minutes on the step mill and you will be dripping in sweat. That's a promise.
If you want to try HIIT cardio, here is a good article for you to read that explains the differences between HIIT cardio and LISS -> http://www.evolutionary.org/hiit-sprints-vs-liss-cardio