Like push ups, you will need to think about your physical location before choosing sit ups as one of your HiiT exercises. You don’t need any specific equipment, but an area of grass or sand will be more comfortable.
Have a look at these links to see some sit up examples and variations. http://www.bodybuilding.com/exercises/detail/view/name/sit-up http://fitness.mercola.com/sites/fitness/archive/2014/05/16/crunches-abdominal-exercises.aspx