Doing sit-ups on the floor or on a slant board isn’t a problem for most experienced lifters, but try doing them hanging upside down using old-school inversion boots. Prepare to be humbled!
Another cool exercise you can do with inversion boots is inverted squats. Instead of just bending at the hips as you would with sit-ups, try bending at the hips and knees.
You can assist yourself if necessary by pulling on yoga straps suspended from the chin-up bar on either side of you, or simply by pulling on your thighs. Make sure to lower yourself down to the start position under control without any assistance. Sequence this movement between sets of squats or deadlifts for the best effect.