A balloon, a sky full of birds and planes, and a trail of gold coins in between. Boost Balloon is about staying aloft as long as possible. A blue shield guards you for the first five seconds; after that, hitting an obstacle or touching the edge of the screen damages the balloon, while the basket itself cannot be hurt.
Short bursts give far finer control than holding the boost down, so tap the button rather than leaning on it. Geometry Lite relies on the same sort of tap timing.