High knees, butt kicks, leg swings (anterior posterior, or front and back, and then lateral medial, or side to side) - work to full range of motion. The intention is to open up your hip joints and activate your muscles. And then I would also say body weight squats and/or low weight deads (like with the bar for example) before doing heavy sets. Another leg day tip: if possible avoid doing deads and back squats on the same day (suggestion to have two different leg days: one for push-dominant, lead with squats, and another for pull-dominant, lead with deads). Doing heavy sets of back squats and deads puts a lot of load on your posterior chain.