Here are some ways to turn a hill into a workout.
By making small adjustments to the way you go up and down a hill, you can not only make it more fun but you can turn it into a nice workout.
Going uphill is a concentric exercise that engages the muscles at the front of your thighs (your quadriceps) as well as your glute muscles a lot more than just walking on level ground.
Going downhill is mostly an eccentric exercise, which lengthens the muscles under load (basically by applying the brakes). Eccentric exercises can create more muscles soreness which is why I suggest trying some of these variations.
Try some of these the next time you are out for a walk and encounter a nice hill. By doing a few repeats of these variations, you can have some fun and build some strength, stamina and get your heart rate up in a hurry.