I'm pretty biased, being an ex rower, but I don't think you can look past the rowing machine. It is often claimed that about 70% of the power in your stroke comes from the legs, and the majority of the rest from your back. Using the machine correctly and you will really hit the legs and get some very good HIIT in.
Try something simple like this:
20 seconds ON, 40 seconds rest. Repeat x 15. Sorted!