How to be efficient with large blocks of time?
ff254's comment has a great list of links and I think reading papers is the best academic use of that time time. But I also recommend using the time to relax so your don't get burnt out from work/class. Two hours is perfect amount of time to grab a meal, read a book, take a nap, or hit up the gym. Remember to invest in your mental and emotional wellbeing as well as your academic education.

(This was originally a comment, but it got a little long so now it's an answer. I'm sorry it's not directly answering the question.)