There's no substitute for football workouts that take place on the gridiron. No matter how many stairs you master, or how much iron you pump, field workouts will still directly affect your game like nothing else. The best way to improve how quickly you get up after being knocked down, or how fast you react to something you see, is to literally recreate it on the field.
Jump lunges: This will increase leg and groin strength while improving stride flexibility. Begin by taking one lunge forward, stopping when your back knee is an inch from the ground, and your front thigh is parallel to the ground. Hold this position for three seconds, then push off your front leg, jumping high into the air while someone throws a football above your head at your peak height. Catch the ball, and repeat the movement 25 times.
Jump squats: This will increase leg strength and jumping height. Do a standard squat, and hold the "down" position for three seconds, then jump high into the air while someone throws a football above your head for you to catch. Perform 25 repetitions.
Lateral hops: This will increase leg strength and speed. Place something small on the field, and jump from side to side over the object. Have someone throw a ball over your head for you to catch randomly, to practice reaction speed. Perform 25 repetitions.
Lines: This will improve how well you change direction, and your quickness over a short distance. Line up on the goal line in your "down" position. Sprint to the 10-yard line and back, then sprint to the 20 and back, to the 30 and back, to the 40 and back, and finish by sprinting to the 50.
Six cones: This will improve your change-of-direction quickness and speed over a short distance. Stagger six cones over 60 yards, with 10 yards between each. Sprint from cone to cone, planting and changing direction each time. Skill-position players should carry a ball, and shift possession to the "outside hand" during each change of direction. Linemen should perform a swim or rip move each time. Do five sets of these.
Gut buster: This will improve your "get up" quickness, and overall endurance. Start by doing 20 up-downs at the goal line, running in place when standing. Sprint to the 50, stop, then do 20 crunches. Sprint back to the goal line, then do 20 pushups. Continue this for five total sets, while adding in 20 squat jumps and 20 jumping jacks.
Four cones: This will improve your reaction speed and quickness over short distances. Place three cones in a straight line, each separated by 10 yards. Designate the cones as 1, 2 and 3. Set the fourth cone 15 yards away, and get in the "down" position by that cone. Run in place and have a partner yell out random numbers between 1 and 3. Sprint to that cone and back, running in place every time as you wait for the next number. Do three sets of 60 seconds.
Shuffle and hit: This will improve your leg strength and reaction speed. Line up two cones 10 yards apart, and get in the "down" position next to one cone. Have a partner stand in front of you, holding a hitting dummy. When he yells the signal, get out of your stance and hit the dummy, then shuffle sideways to the next cone. Get back in the down position, and wait for his signal. Repeat this in three sets of 45 seconds.