Tabatas will KICK YOUR ASS. Very effective.
Myself, I jump rope. Easier on the joints than running, pretty darn strenuous, very portable, very cheap, difficult to master so it keeps me interested (been at it a few years and still slip up occasionally). For my last fitness test, I scored well on the run (ahead of many guys younger than me) despite having done ZERO running in preparation for the test, which I attribute to my rope skipping.
Myself, I jump rope. Easier on the joints than running, pretty darn strenuous, very portable, very cheap, difficult to master so it keeps me interested (been at it a few years and still slip up occasionally). For my last fitness test, I scored well on the run (ahead of many guys younger than me) despite having done ZERO running in preparation for the test, which I attribute to my rope skipping.