One of my kids greatest motivations is free time. If we get school (or chores) done quickly we have time for fun. Playing, running through the sprinkler on a warm day, going for a bike ride, playing board games, going on adventures (to the zoo, museum, special movie…)
Growing up being HSed extra time was a big motivation! In highschool we were able to work part time because school was done and we still had time for visiting with friends.
I think working toward a goal or prize can help many kids. I know even asan adult when I worked, before kids, working towards a vacation was big motivation. Knowing that working hard would have a reward of vacation days.