Low Impact, Low Intensity?

What're your favourite low impact and low intensity workouts, that still pack a punch?

I feel like I've got myself caught in a loop of doing the same few over and over again.

High impact and high intensity sets me off on flares of my health conditions so I avoid them at all costs and tend to stick to walking workouts, pilates, and strength, but I have only four basic weights of dumbbells and I'm growing out of them as such. While I save for more, what existing workouts do you think are a challenge?

I did Erica's Upper Body Negative Training yesterday and it unlocked a whole new level for me, I even had to drop down weights for some of them. Now I'm wondering what else I've missed!