Get unbiased advice before you buy. Yes Biking, swimming, and running are all pretty decent workouts. I guess they can be somewhat challenging too. But if you want a real total body workout that will have you rolling on the floor in a lactic acid induced stupor, check out Concept II's Ergometer and get ready for a workout experience straight out of hell.
Nothing compares to the intensity of an erg workout.
20 minutes of intense erging a day is enough to wear out even the best athlete. All major collegiate crews use the Concept II Model C as does the US National Team and nearly all junior-rowing programs. The machine creates resistance with a large fan. The level of resistance can be varied to simulate the feel of different types of boats. The machine has a seat on wheels attached to a single track, and the handle connects to the flywheel with a lightweight chain.