try skipping.Try perfecting each variation.Try Ascending Descending Rowing.Basically there's 10 rounds.100m,200m,300m,400m,500m and then vice versa again.Really fries you.Also try see if you can achieve 1000m in under 10min.I should do it again.Last I did it in I think 6-7min.Uphill sprints also works well but you can only do it twice a week with 48hrs reat due to its intense work. Great ab workout also