Imagine Earth

Best done (and possibly has to be done) in Endless Mode.

Make a large planet (largest you can make) and remove all your competitors. Include Gaian Temples. Adjust everything that seems beneficial (starting funds, licences etc...) and start the game.

Without any opponents, your only challenge here it managing global warming. You can see this in the stats bar at the top of the screen, towards the right. If the number here goes negative, problems will occur. I was a little paranoid about global warning as I lost several Challenge Mode games because of it.

Counter it by placing plenty of trees. You can use the Gaian Temples or Seed Bombs.

For Gaian Temples, 'charge' them with goods. Once you have enough, select the option at 9 o'clock in the temple and select a vacant space to create a forest.

Your Park residential areas will create Seed Bombs over time, so be sure to use plenty of them. Find the Seed Bomb and place it in a similar way for more forest.

By doing this, along the way you should get the Forester achievement. Edit: Appears Forester must be done on a campaign world.

Aside from that, take your time. Research items that will reduce the environmental impact of your buildings and keep an eye on the global warming figure. Stop and plant more trees if it is getting low.

Otherwise, just build up your cities and associated resource buildings and you will reach the 10 million mark eventually.