Whenever you are out walking and come across an animal, hit it with an arrow. It's not a huge one time boost in skill, it's a more slow, steady, and always available way to improve your skill. I never miss a chance to stalk and kill an elk or fox. Rabbits are my favorite because the are fast and small, and therefore harder to hit. As for arrows it doesn't take much to kill even the largest wildlife (elk or deer, i consider bears to be more of an an enemy since they are aggressive) take some free steel arrows off the target when you see a guard practicing. Hunting is free, it's always available, it builds on your archery skill, and it helps pass the time when you are traveling on foot. Happy Hunting
WARNING SPOILER!!!
Another way would be the torture victims inside the Dawnstar Dark Brotherhood Sanctuary. I use them to level up my skills often. Just don't forget to heal them. Not only to level up your restoration skill, but they could die before giving you some valuable information.