Hey, terrific! Did you enjoy the book itself? I really liked AoPS series; I remember getting a whole lot out of Introduction to Counting and Probability. I distinctly remember thinking over and over, why didn't they tell me this in school!

I'm not sure what your ultimate goals are, and you're probably already familiar with the online AMC8/10/12 and Olympiad problem banks that others have mentioned; if not, I couldn't recommend them more highly for bringing newly learned skills to bear on novel problems. At one point, I worked through all the AMC8, then all of the AMC10. It became very addictive!

Also, perhaps more recreationally, the MAA does a question of the day called Minute Math, which is always good for a quick diversion.