I'm currently a graduate student in engineering (working toward my Master's). Over the summer semester I'm taking three classes in addition to researching at my adviser's lab.
The lab is located off-campus (about a 30 minute commute), so it doesn't make much sense to fill in all of my gaps between classes with research time.
However, after making my schedule, I realize that I have these awkward 2-hour breaks in the afternoon. I also have 30-minute breaks later in the day, as well.
Given that I can't research in these times (the commute is too long), and given that I'm generally a nocturnal person and like to work on problem sets at night (from the comfort of my dorm), what is the best academic use of my time during these breaks?
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.)