Caporegime
Skipping is fine, step ups on a box are fine, jogging on the spot is fine (though highly boring). 



Can I ask a quick HIIT question save me starting a new thread, Can I substitue skipping for my running HIIT on days when its raining heavy outside (and keep the same intervals) or is it best to stick to one type for best results?
level 15 on bike on hill setting isn't really going to improve your cardio. Ive been in rehab for 6 months doing phyis (none impact) and thats defiantly really to high you want to be at 140+ rpm on the bike so level 8-10 depending on your fitness. if thats to easy do 160 rpm find it hard to belive you are training cardio on level 15 when you hit 9.1 on a bleep test.

Depends on the type of results you're looking for? Increasing your speed at running, then yeah it would be. Losing weight, then mix it up a bit can be more of a shock to the body.

