I spent a holiday (mostly)without internet... and I didn't have a mental breakdown. :V
Read books, study for tests/quizzes/exams, talk to my idiot siblings, read my textbooks, attack some assessment books, try to figure out the chords on my guitar, Organize my desk, draw, reorganize my desk, repack my stuff, charge my laptop and other electronics, mark my calendar, read 'Reader's Digest' over and over and over again, do all my homework in advance, read other textbooks and my parents' encyclopedias/dictionaries/collections of writing... etc.