Being a greener person

It is apparently "Green GB Week". The cynical among us may say this is a publicity stunt to distract from the a certain summit happening on Wednesday which seems likely to go horribly wrong, it's as good a time as any to reflect on the state of our world. Some assumptions I'm not interested in debating here:

1. Climate change is happening.
2. Climate change is significant.
3. Climate change is primarily man-made.
4. We have an ethical duty to do something about it.

Given all that, the question on my mind is "how can I, a moderately intelligent, moderately well-off person make the largest positive difference to the environment?" For anyone that doesn't know me, I already cycle to work every day so that takes out one of the obvious changes I could make. I also have two children; I'm well aware this is in many ways a significant increase in "my" environmental cost but it isn't something which is going to change. It also means that I am time-poor. I am prepared to invest potentially non-trivial amounts of money in something if it could significantly affect my environmental cost. Not sure what "non-trivial" means yet.

So... tell me what things I can do which will have the largest benefit to the environment. If they just affect me and my immediate family, that's OK but what would be wonderful would be things which affect a larger group of people. Particular attention will be paid to ideas with quantitative numbers behind them - feel free to refer to the amazing "Sustainable Energy - without the hot air" to get those numbers. Stuff which I've considered:

1. Going (mostly) veggie. I know I should but... I like meat.
2. Voting Green. This is a tricky one - firstly because our electoral system means a green vote is almost certain to be "wasted" (I live in a Con-Lab marginal), and secondly because I have serious problems with the Greens' energy policy. "Do it all with renewables" just doesn't add up (see "without the hot air" for the numbers); as far as I can see, the only way we can in the short term get low carbon energy generation is with nuclear. It's not perfect, but it's the least bad option for base load generation right now.

What else can/should I do?