A good cup of tea. Sometimes I treat myself and make tea using filtered water. It makes a huge difference, especially with good quality tea leaves, like, say, ones sent to you from Hong Kong by jlrpuck.
Cabin Pressure. Which is pretty much the most amazing thing. And not just because it features Benedict Cumberbatch's jaguar-cello voice, but because it is genuinely hilarious and makes me giggle like an insane person during my commute.