You want to work those legs...then hiking is the best thing you can do here! Lots of different terrain, different difficulties of hiking trails. You pick, it's all here.
Good hiking shoes are required. Water, water and water! Always take lots of that and some food.
This park is a great place in the winter too. You can experience real backcountry skiing, snowboarding, snowshoeing and cross country. The Diamond Head area and the Black Tusk/Garibaldi Lake area are mainly used. You can go from short trips to overnight trips. The shelters at Garibaldi Lake and Elfin Lake are available for sleeping or you can just put your tent in the snow.