Some of my favorite marathons are perennial classics like The Twilight Zone, Star Trek, or The Honeymooners. Even though I've seen all the episodes countless times, I never seem to tire watching them again.
I remember once in my teenage years, I was really sick one summer and I whiled away the hours tuning in to a marathon of episodes of The Incredible Hulk on the Sci-Fi Channel (back before it changed its name to Syfy).
Nowadays, shows are collected in DVD packages and we can enjoy a marathon viewing of a particular season or an entire series if we're being bold and have the leisure time to do so. Broadcast marathons are still great to stumble upon, especially if we don't have that particular show in our DVD collection. Batman is a great example.
So if your holiday plans get washed out when the weather doesn't cooperate, or if you intentionally want to have a quiet day off, flip the channels and you're bound to come across a fun television marathon of a great show.