Mountain climbers are already solid, but a few variations make them way more effective: the classic straight-leg mountain climber for overall cardio and core, cross-body (bringing knee to opposite elbow) for better oblique engagement, slow mountain climbers for core control (these burn more than they look), and spider climbers where you drive your knee outside toward your elbow for hip mobility. I personally like mixing fast reps with slow controlled ones. It hits both endurance and core strength without needing extra equipment.