Go to the library! Or go to a friend's who lives on campus! Also, some societies are on during the day, you could to them if it doesnt clash with your lectures!
I go to the library and catch up with some work or reading, as I'm not very good at working once I get home.. procrastination takes over! So working in the library between lectures is just the perfect time for it.
Depending on what time it is, it's usually time for a meal once the first seminar is done. So we just get food and hang out...or if we're travelling between campus' we eat on the move
I guess I'm going to do what most have mentioned here, either trying to get some work done or procrastinate in front of a computer, do some socialising or... maybe a quick nap in a dark corner?
Speaking of dark spots, pool would certainly be an idea as well. If I could only play it well...